Astronomy

How old is the oldest light visible from Earth?

How old is the oldest light visible from Earth?


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Because light can only travel so fast, all of the light we see in the sky was emitted at a previous moment in time. So if for example we see a supernova or some other great stellar event, by the time we see it, it maybe long over. That made me kind of curious, what is the most ancient light we can see from earth?

The universe is supposedly ~13+ billion years old, but we are probably not at the very edge of the known universe so all the light we see is probably less than 13 billion years old. So what is the oldest light we can see? and as an optional follow-up question how do we know the age of that light?

I guess the light itself may not actually be literally 'old', but its probably obvious what I'm asking here, put another way: what's the longest distance that now earth visible light emitted has traveled to reach the earth? Though that reformation of the question gets kind of tangled with lensing effects.


The oldest light in the universe is the cosmic microwave background. Roughly 380,000 years after the Big Bang, protons and electrons "recombined"1 into hydrogen atoms. Before this, any photons scattered off the free electrons in the plasma filling space, and the universe was essentially opaque to light. Once recombination occurred, however, photons were able to "decouple" from the electrons and move through space unimpeded. This relic radiation is still observable today; it has been redshifted and cooled.

We can detect light from very distant objects, and we have. It makes more sense to talk about distance in terms of redshift; the larger the redshift, the farther away an object is. There are a number of extremely high-redshift objects, some of which have had their measurements confirmed, and others of which have not. Candidates include

  • MACS0647-JD (redshift $z=10.7^{+0.6}_{-0.4}$), a very distant galaxy.
  • UDFj-39546284 (redshift from $z=10$ to $z=12$), another galaxy or protogalaxy (although its distance is uncertain, and its exact nature is disputed).
  • GN-z11 (redshift $z=11.09^{+0.08}_{-0.12}$), a very luminous distant galaxy.

All of these objects would have formed some hundreds of millions of years after the Big Bang, however, so the light we see from them is much "younger" than that of the cosmic microwave background.


1 I've never liked the usage in this context, as this was the first time they combined; the "re" is kind of misleading.


This April 2, 2018, CNN article says:

Scientists detect 'fingerprint' of first light ever in the universe"

Following the Big Bang, physicists believe there was only darkness in the universe for about 180 million years, a period known by scientists as Cosmic “Dark Ages.”

So I think your answer might be that Big Bang + 180 million years is the oldest light we can see.


What's the oldest light that we can see?

The Cosmic Microwave Background is considered to be the oldest E-M radiation detectable to us. It's in the microwave spectrum, so it can't be seen with the naked eye but is picked-up by "radio telescopes". We call it "light" in the broad sense.

One remarkable aspect about this background radiation is its near-uniformity in all directions. Astronomers reason that the uniformity is too strong for the source to be a really big thing like a huge balloon… but that would be the case if it was all actually as far apart as it seems to be.

If it were really as big as it looks, it would take twice the age of the universe for one side to be affected by the other side! Instead, astronomers believe that what we see was a very small body, which has become bigger; that's why it looks the same in each direction. Some of the growth is called metric expansion of space and has a different meaning than ordinary growth.

How do we know the age of that light?

The age of the cosmic background light can only be determined indirectly, first by knowing how long ago the Big Bang happened, then by figuring when the light was emitted in the course of the Big Bang.

By comparing the rate at which everything seems to be getting bigger with how big everything seems to be, in the same way that you might estimate how long it would take to drive to a place given the speed of the road and the distance, we calculate the Hubble Constant. This helps us calculate how long ago the Big Bang happened.

Also, there are certain "sound waves" (baryonic acoustic oscillations) where old things we see, including the cosmic microwave background, get brighter and dimmer with a rhythm, like a clock's pendulum. They can be measured either left-right (for moving things) or by monitoring a video (for stationary things). Measuring these rhythms and comparing them to the Hubble Constant also helps to calculate how long ago the Big Bang happened.

Finally, the microwave background has physical qualities (like temperature and density) that make it possible for us to determine when it was emitted during the expansion and cooling of the Big Bang. Together using all these calculations is how we figure the age of the cosmic microwave background light.

Astronomers believe that this combined calculation (called "LCDM", "Lambda-CDM", or "Big Bang Cosmology") is very good because the different numbers do line up, for the most part*. They were pleased to report more good findings as recently as 2018 when a study called the Dark Energy Survey finished. Nevertheless, since LCDM includes certain assumptions that may never be validated, and since there are still some unexplained discrepancies, we don't know whether another kind of calculation would be better, provided that it still fits the measurements.

How do we know that this is the oldest light?

It is only by thinking about the physical qualities of the cosmic microwave background, and thinking about when during the Big Bang it must have emitted its light, that astronomers identified it as the oldest possible light in the universe, older than any stars or galaxies. It doesn't tell us how old it is by itself; in fact, astronomers are always making sure that it's not in fact just a layer of dust on the telescope!

How far away is the cosmic microwave background?

This is a really hard question to answer. According to Big Bang Cosmology, the cosmic microwave background wasn't "somewhere" but instead it was everywhere. And the distance it has traveled since the Big Bang is different than the time multiplied by light speed, because of metric expansion of space. This is a result of the relativistic length-contraction due to the speed at which everything is moving.

Is the observable universe younger than the greater universe, assuming that that exists?

Calculating the amount of time from the Big Bang to now gives the same result whether you consider our observable universe or the greater universe that may exist. That's why the age of "our" universe is the same as the age of "the" universe.

*Some different studies to determine the Hubble constant have given cosmologists pause (link 1, link 2); depending on which part of the universe you look, it may be close to 67 or it may be closer to 73 in the standard units.


Scientists have discovered a galaxy, named GN-z11 (already mentioned by HDE 226868), which existed a mere 400 million years after the Big Bang, or about 13.3 billion years ago:

Farthest Galaxy Yet Smashes Cosmic Distance Record

The discovery of a 10 billion year old star was announced just last week:

Hubble spots farthest star ever seen

Here is a list of distant astronomical objects on Wikipedia.


You've made two questions using semantics:

  • "How old is the oldest light visible from Earth?"

From @Pela's answer to: Why is there a difference between the cosmic event horizon and the age of the universe? - So in ~100M years the most distant light will reach us, from over 116M light years away.

The 16 Gly that the distance to the event horizon is today is sort of a coincidence. It has nothing to do with the age of the Universe. It only depends on the future expansion of the Universe, which in turn depends on the densities of the components of the Universe (Ωb, ΩDM, ΩΛ, etc.). If the Universe has been dominated by matter (or radiation), then there would be no event horizon: No galaxy, ever-so far away would not be visible to us, if we just had the patience to wait. A galaxy is 10,000 billion lightyears away? Just wait long enough (exactly how long depends on the actual density).

However, our Universe happens to be dominated by dark energy, which accelerates the expansion without boundaries. This unfortunately means that the light leaving today from a galaxy 17 Gly away will be carried away by the expansion faster than it can travel toward us. In contrast, the light emitted today from a galaxy 15 Gly away will travel in our direction, but will nonetheless initially move away from us due to the expansion. However, its journey toward us makes this expansion rate smaller and smaller (since the expansion rate increases with distance from us), and after a period of time it will have traveled so far that it has overcome expansion and starts decreasing its distance from us and eventually reach us after 100 Gyr or so.


  • "I guess the light itself may not actually be literally 'old', but its probably obvious what I'm asking here, put another way: what's the longest distance that now earth visible light emitted has traveled to reach the earth? Though that reformation of the question gets kind of tangled with lensing effects?"

Yes, that's a totally different question…

See one of the earliest papers: "Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe" by Davis and Lineweaver (2003).

Newer works:

"The Shared Causal Pasts and Futures of Cosmological Events", by Friedman, Kaiser, and Gallicchio (2013).

"Conclusion:… While the observable spatial densities of galaxies, clusters, and thus quasars are thought to reflect correlations set up during inflation, it remains an open question whether inflationary era events at specific comoving locations - where quasar host galaxies later formed - could yield an observable correlation signal between pairs of eventual quasar emission events at those same comoving locations billions of years after the inflationary density perturbations were imprinted.

In closing, we note that all of our conclusions are based on the assumption that the expansion history of our observable universe, at least since the end of inflation, may be accurately described by canonical general relativity and a simply-connected, non-compact FLRW metric. These assumptions are consistent with the latest empirical search for non-trivial topology, which found no observable signals of compact topology for fundamental domains up to the size of the surface of last scattering.

FiG. 1. Conformal diagram showing comoving distance, $R_0χ$ in Glyr, versus conformal time, $R_0τ /c$ in Gyr, for the case in which events A and B appear on opposite sides of the sky as seen from Earth (α = 180°). The observer sits at Earth at $χ = 0$ at the present conformal time $τ = τ0$. Light is emitted from A at ($χA, τA$) and from B at ($χB, τB$); both signals reach the Earth along our past lightcone at ($0,_{τ0}$). The past-directed lightcones from the emission events (red and blue for A and B, respectively) intersect at ($χAB, τAB$) and overlap for $0 < τ < τAB$ (purple region). For redshifts $z_A = 1$ and $z_B = 3$ and a flat ΛCDM cosmology with parameters given in Eq. (11), the events are located at comoving distances $R_{0χA} = 11.11$ Glyr and $R_{0χB} = 21.25$ Glyr, with emission at conformal times $R_{0τA}/c = 35.09$ Gyr and $R_{0τB}/c = 24.95$ Gyr. The past lightcones intersect at event AB at $R_{0χAB} = 10.14$ Glyr at time $R_{0τAB}/c = 13.84$ Gyr, while the present time is $R_{0τ0}/c = 46.20$ Gyr. Also shown are the cosmic event horizon (line separating yellow and gray regions) and the future-directed lightcones from events A and B (thin dashed lines) and from the origin (0,0) (thick dashed lines). In a ΛCDM cosmology like ours, events in the yellow region outside our current past lightcone are space-like separated from us today but will be observable in the future, while events in the gray region outside the event horizon are space-like separated from observers on Earth forever. Additional scales show redshift (top horizontal axis) and time as measured by the scale factor, $a(τ)$, and by proper time, $t$, (right vertical axis) as measured by an observer at rest at a fixed comoving location.

Also see: "Causal horizons in a bouncing universe", by Bhattacharya, Bari, and Chakraborty (2017):

"Conclusion: The present work shows that the causality problem in bouncing universe is intrinsically related to an understanding of the various phases of the universe during the contraction phase. As our understanding of the contraction phase is purely speculative at present the models we use to figure out the nature of particle horizon remains over simplistic. The present authors believe that although the causality problem in bouncing universe models are far from being solved the present article shows the qualitative and quantitative difficulties one must have to circumvent in the future to produce more meaningful results.".

Short answer: It's 46.9B light years. Another Wikipedia page says: 46.6B light years. The experts above calculate 46.2.


Light and Astronomy

When stargazers go outside at night to look at the sky, they see the light from distant stars, planets, and galaxies. Light is crucial to astronomical discovery. Whether it's from stars or other bright objects, light is something astronomers use all the time. Human eyes "see" (technically, they "detect") visible light. That's one part of a larger spectrum of light called the electromagnetic spectrum (or EMS), and the extended spectrum is what astronomers use to explore the cosmos.


A new look at the universe’s oldest light

View larger. | In 2013, the Planck space telescope released the most detailed map to date of the cosmic microwave background, the relic radiation from the Big Bang. It was the mission’s first all-sky picture of the oldest light in our universe, imprinted on the sky when it was just 380,000 years old. Now a new, independent study agrees with Planck’s results. That’s good news for astronomers trying to pin down the universe’s age and rate of expansion. Image via ESA.

How old is our universe? That’s one of humanity’s oldest and most fundamental questions. Now, astronomers using the Atacama Cosmology Telescope (ACT), high in the Andes of northern Chile, have announced a new measurement of the cosmic background radiation, our universe’s oldest light, discovered in 1965 and sometimes called an echo of the Big Bang. The new measurement suggests the universe is 13.77 billion years old, give or take 40 million years. This new result agrees strongly with results from the European Space Agency’s Planck satellite, which measured the cosmic background radiation from 2009 to 2013. And it agrees with what’s called the Standard Model of particle physics, developed in the 1970s and refined in the years since then. The Standard Model encapsulates scientists’ best current understanding of how elementary particles and the fundamental forces of nature relate to one another.

The new peer-reviewed results were published in the Journal of Cosmology and Astroparticle Physics on December 30, 2020.

Agreement between independent studies gives scientists confidence their work is correct. Non-agreement does the opposite: it makes scientists think their ideas need more work. In 2019, a study had suggested the age of the universe might be hundreds of millions of years younger than Planck’s data had indicated. The new results from the Atacama Cosmology Telescope, on the other hand, agree with Planck’s results. Simone Aiola of the Flatiron Institute’s Center for Computational Astrophysics – a co-author on the new study – commented:

Now we’ve come up with an answer where Planck and ACT agree. It speaks to the fact that these difficult measurements are reliable.

Like the Planck satellite, the Atacama Cosmology Telescope (ACT) in Chile also studies the cosmic microwave background. Here’s a portion of a new ACT image, covering a section of the sky 50 times the width of a full moon. This image represents a region of space 20 billion light-years across. Image via ACT Collaboration/ The College of Arts and Sciences.

The new measurements don’t just tell us the age of the universe. They also suggest how fast the universe is expanding.

The rate at which the universe is expanding is described by what’s called the Hubble Constant. Writing at his blog on January 6, 2021, astrophysicist Brian Koberlein described the Hubble Constant in the context of the cosmic microwave background in this way:

In the early universe, there were small fluctuations of density and temperature within the hot dense sea of the Big Bang. As the universe expanded, the fluctuations expanded as well. So the scale of fluctuations we see in the cosmic microwave background today tells us how much the universe has grown. On average, the fluctuations are about a billion light-years across, and this gives us a value for the rate (the Hubble parameter) as somewhere between 67.2 and 68.1 kilometers per second per megaparsec.

The new data from ACT gives a Hubble constant of 67.6 kilometers per second per megaparsec. The Planck researchers had previously estimated 67.4 km per second per megaparsec. Steve Choi at Cornell University, first author of the new paper, said:

I didn’t have a particular preference for any specific value it was going to be interesting one way or another. We find an expansion rate that is right on the estimate by the Planck satellite team. This gives us more confidence in measurements of the universe’s oldest light.

Steve Choi at Cornell University, lead author of the new study. Image via Cornell University.

It’s worth noting, however, that – though the values for the Hubble Constant derived from data by Planck and ACT agree well – neither agrees well with values for the Hubble Constant derived via distant variable stars and supernovae (exploding stars). Brian Koberlein explained:

… you can use variable stars and distant supernovae to create a cosmic distance ladder that tells you the rate of expansion. The problem is, this alternative method gives a larger value for the Hubble parameter. If the supernova method is right, then the universe is younger and has expanded more quickly than the [work on the cosmic background radiation] seems to support. For a while, the hope has been that new observations and new methods of measuring cosmic expansion would solve this problem …

But, Koberlein said, the new study from ACT dashes those hopes.

According to Michael Niemack, a co-author of the new paper:

The growing tension between these distant versus local measurements of the Hubble Constant suggests that we may be on the verge of a new discovery in cosmology that could change our understanding of how the universe works. It also highlights the importance of improving our measurements of the cosmic microwave background with the Atacama Cosmology Telescope as well as the future Simons Observatory and CCAT-prime projects that we are now building.

The universe is ancient, but how ancient? The current answer appears to be 13.77 billion years, give or take 40 million years. Image via NASA/ WMAP Science Team/ Wikipedia.

To calculate the universe’s age, scientists need to estimate how far light from the cosmic microwave background (CMB) – the “afterglow” of the Big Bang and the measurable record of when photons first escaped the “fog” of the early universe – has traveled to reach Earth. But that is not an easy task, so astronomers use triangulation by measuring the angle in the sky between two distant objects. With Earth as the third point of the triangle, they can then estimate the distance of both objects from Earth.

ACT is able to measure slight fluctuations in the cosmic microwave background with great precision, which helps to refine the estimates. Suzanne Staggs at Princeton University commented:

The Planck satellite measured the same light, but by measuring its polarization in higher fidelity, the new picture from the Atacama Cosmology Telescope reveals more of the oldest patterns we’ve ever seen.

Stunning view of some of the oldest known galaxies in the universe, from the Hubble Space Telescope in 2015. Image via ESA/ NASA/ Smithsonian Magazine.

Bottom line: New observations of the oldest light in the universe indicate that the cosmos is 13.77 billion years old, and help resolve inconsistencies with other previous estimates.


University of California, San Diego Center for Astrophysics & Space Sciences

Stonehenge, constructed between 3100-2000 BCE on England's Salisbury Plain, may have been a Stoneage astronomical site (observatory is too strong a word), at least in part. Certainly the alignment of the "heelstone" with the rising Sun on Midsummer's Day (June 21, the Summer Solstice) represents a true astronomical alignment, and many other Megalithic sites have similar alignments. In Stonehenge Decoded, astronomer Gerald Hawkins argued that there exist a large number of astronomical alignments, though further study suggests that many of these are fortuitous.

Cosmologist Fred Hoyle has suggested that Stonehenge may have been used to keep track of the solar-lunar eclipse cycle. Far outside the still partially standing ring of Sarsen Stones is a ring of 56 holes, known as the Aubry holes. Hoyle has noted that movement of a marking stone by 3 positions each time the Sun rose over the heelstone (or by one position three times yearly) would complete a circle in 18.67 years -- approximately the period for the "nodes", the intercepts of the lunar and solar paths in the sky, to complete a cycle. Certainly ritual use of Stonehenge would have been more important that its astronomical functions and much of this interpretation must remain speculation. We may be certain, however, that Stonehenge was indeed constructed by Stoneage humans without the assistance of alien astronauts as suggested in some pseudo-scientific books. Visit the Complete Stonehenge

Eastern observers, notably the Chinese, kept careful track of events in the skies, particularly the appearance of "guest stars" -- comets, novae and other transients. Chinese records of the guest star that we now call Comet Halley can be traced back to 240 BCE and possibly as early as 1059 BCE. One of the most important Chinese records is of a guest star that was bright enough to be seen during the daytime for nearly a month in the constellation that we call Taurus in July 1054. We believe this to be the supernova explosion that gave rise to the Crab Nebula, and our knowledge of the date of the explosion itself is a very important key in understanding the deaths of massive stars. This event was also chronicled by the Anasazi in Chaco Canyon and by Native Americans elsewhere, but is curiously absent from European records in the Middle Ages.

As the above suggests, Archaeoastronomy is an active and exciting field of research.

Western scientific history begins with the ancient Greek civilization about 600 BCE.

The Ionian region of Asia Minor appears to have been a site of particular philosophical/scientific/mathematical activity for several centuries.

We will review the progress of science by highlighting a few key natural philosophers, scientists and mathematicians. As Isaac Newton said,"If I have seen further, it is by standing on the shoulders of Giants."

Pythagoras of Samos (

Pythagoras developments in astronomy built upon those of Anaximander from whom, apparently, came the idea of perfect circular motion. The Pythagoreans believed that the planets were attached to crystalline spheres, one for each planet, which produced the Music of the Spheres. These spheres were centered on the Earth, which was itself in motion. Pythagoras is also credited with recognizing that the "morning star" and "evening star" are both the planet Venus.

Aristotle (384-322 BCE)

Aristarchus of Samos (

Eratosthenes of Cyrene (276-197 BCE)

Claudius Ptolemy (

Ptolemy's Geography remained the principal work in that field until the time of Columbus.


Copernicus Heliocentric Solar System vs. Ptolemy's Geocentric Model
Both models employed perfect circular motion with epicycles, equants .

Nikolas Kopernig (Copernicus, 1473-1543)

Tyge (Tycho) Brahe (1546-1601)

Galileo Galilei (1564-1642)

  • development of the concept of inertia, later refined by Newton.
  • a variety of experiments on falling bodies which demonstrated that the acceleration of gravity is independent of mass. There is no evidence that Galileo actually dropped objects from the Tower of Pisa. Rather, his experiments were conducted with an inclined plane as shown in this animation.
  • the first Theory of Relativity, valid for velocities much smaller than the speed of light.
  • sunspots on the Sun and craters and mountains on the Moon.
  • The so called "Galilean satellites" which orbit Jupiter -- Io (with the volcanos), Europa, Callisto and Ganymede. Here's more on Jupiter and her satellites from the Siderius Nuncius and an animation showing what Galileo observed.
  • rings of Saturn.
  • the phases of Venus.

Johannes Kepler (1571-1630)

  1. The orbits of the planets are ellipses with the Sun at one focus.
  2. The planets sweep out equal areas during equal times of the orbit.
  3. The square of the orbital period is proportional to the cube of the planet's distance from the Sun. (If you measure the period in Earth years and the distance in Astronomical Units (1 A.U.= the average distance of the Earth from the Sun), then Period 2 = Distance 3 .)

Here's a page with some nice animations of Kepler's Rules, and here is another way to play with them.

Obviously Kepler's Rules require that the Sun be the center of the Solar System, in contradiction with the Aristotilean ideal. The first rule eliminates the circular motion which had been fashionable for 2 millennia. The second replaces the idea that planets move at uniform speed around their orbits,with the empirical observation that the planets move more rapidly when they are close to the Sun and more slowly when they are farther away. The third rule is a harbinger of the Law of Gravitation which would be developed by Newton in the latter part of the 17 th century.

Isaac Newton (1642-1727)

Other pioneers and milestones in the advance of Science:

  • 18th Century, William Herschel discovered Uranus, a new planet beyond Jupiter. Barely visible with the unaided eye, Herschel made the observation with his telescope .
  • Early in the 19th Century Adams (English) & LeVerrier (French) independently calculated that there must be another planet beyond Uranus that was producing small gravitational disturbances in Uranus' orbit. First observed in 1846 by Hohan Galle, it was named Neptune. (It was actually spotted earlier by Challis in Cambridge, but Challis did not note his discovery until Galle reported his observation.)
  • 1930 Clyde Tombaugh discovered Pluto.
  • 1910 Harlow Shapley estimated the size of the Milky Way.
  • W. H.Pickering and Annie J. Cannon calculated the surface temperatures of the stars.
  • Einstein (1905) developed the Theory of Special Relativity, based upon the idea that light travels at the same speed in all frames of reference. Modified Newton's Theory of Gravity by developing the General Theory of Relativity (1916).
  • Cecilia Payne-Gaposchkin & Henry Norris Russell determined the composition of stars.
  • 1924 Edwin Hubble established that the Andromeda nebula and other "spiral nebulae" are star systems like the Milky Way at great distances.
  • 1929 Hubble & Milton Humason discovered that the Universe is expanding.
  • 1938 Hans Bethe determined that the Sun's energy comes from thermonuclear fusion reactions.
  • 1940s Karl Jansky observed that the nucleus of the Milky Way and other celestial objects are strong sources of Radio Waves in 1931. Based on radar technology developed in WWII, Radio Astronomy becomes an active field in the late 1940s.
  • 1948 George Gamov developed the Hot Big Bang Theory of the origin of the Universe.
  • 1950's chemical composition of the stars stars build the heavy elements via nuclear fusion reactions, mapped out in a famous paper by Burbidge, Burbidge, Fowler & Hoyle.
  • 1954 Radio Galaxies
  • 1960-63 Quasars
  • 1960s X-ray & Infrared astronomy
  • 1965 Arno Penzias and Robert Wilson from Bell Laboratories discovered the cosmic microwave background radiation remnant of the Big Bang.
  • 1968 Jocelyn Bell (Burnell) & Anthony Hewish discovered Pulsars
  • History of Astronomy at U. Bonn, maintained on behalf of IAU Commission 41 - The History of Astronomy. History of Astronomy & Archaeoastronomy Links.
  • History of Mathematics at St. Andrews U., Scotland, with 1350 biographies & links, including many Astronomers & Physicists. at Rice U.
  • The Art of Renaissance Science
  • History of High-Energy Astrophysics
  • Calvin Hamilton's History of Space Exploration - part of his Views of the Solar System

Prof. H. E. (Gene) Smith
CASS 0424 UCSD
9500 Gilman Drive
La Jolla, CA 92093-0424


Last updated: 16 April 1999


What's the oldest thing we've seen through a space telescope?

Before we get into the meat of the question, let's start off by explaining why we're asking about the oldest thing ever spotted through a telescope. The set-up sounds suspiciously like an astronomy-themed vaudeville routine or something: "I looked through the Hubble Telescope and saw my mother-in-law waving!"

"Hey," you might be thinking, "don't you mean the farthest thing you've seen through a space telescope? How can we see old stuff through a telescope?"

Turns out, the farthest thing you can see in your telescope is the oldest thing as well. (That is, if "you" are an astronomer and "your telescope" is something like the Hubble.) While light speed is the fastest thing in the universe that we know of, it still takes time to travel. When we catch a glimpse of our sun, for instance, we're actually seeing what it looked like eight minutes before, as its rays take eight minutes to reach us. Much, much farther distant stars, planets and galaxies are no different if a star is 20 light-years away, we're looking at it as it shone 20 years ago. Looking at a galaxy 100 million light-years away means we're seeing it not as it looks right this second, but as it looked when dinosaurs were stomping around Earth.

So the farthest thing we can see in our universe is quite necessarily the oldest. Now considering our universe is about 13.7 billion years old, how old do you think is the oldest thing we've seen? Three hundred million years old? One billion?

Try 13 billion. Which really makes you feel the need to tip your hat to telescope technology, no? The Hubble Space Telescope was able to peer 13 billion light-years away to find seven galaxies, some born just around 400 million years after the universe's inception [source: NASA]. Hubble stared at a certain spot (the Ultra Deep Field) for 100 hours, peering at the sky in infrared in order to catch objects the farthest away [source: Plait].

What it found was galaxies ranging from 13 billion to 13.3 billion light-years away. While the numbers aren't confirmed, the initial results are still pretty stunning. Also note that we're already assuming we can top them when the James Webb Space Telescope launches in 2018: The JWST has infrared capabilities that can peer even farther than Hubble and give us a more accurate picture. Even cooler, the JWST should be able to show us light sources that originated just 200 million years after the Big Bang [source: Masetti].


Astronomers agree: Universe is nearly 14 billion years old

Credit: CC0 Public Domain

From an observatory high above Chile's Atacama Desert, astronomers have taken a new look at the oldest light in the universe.

Their observations, plus a bit of cosmic geometry, suggest that the universe is 13.77 billion years old—give or take 40 million years. A Cornell University researcher co-authored one of two papers about the findings, which add a fresh twist to an ongoing debate in the astrophysics community.

The new estimate, using data gathered at the National Science Foundation's Atacama Cosmology Telescope (ACT), matches the one provided by the standard model of the universe, as well as measurements of the same light made by the European Space Agency's Planck satellite, which measured remnants of the Big Bang from 2009 to '13.

The research was published in the Journal of Cosmology and Astroparticle Physics.

The lead author of "The Atacama Cosmology Telescope: A Measurement of the Cosmic Microwave Background Power Spectra at 98 and 150 GHz" is Steve Choi, NSF Astronomy and Astrophysics Postdoctoral Fellow at the Cornell Center for Astrophysics and Planetary Science, in the College of Arts and Sciences.

In 2019, a research team measuring the movements of galaxies calculated that the universe is hundreds of millions of years younger than the Planck team predicted. That discrepancy suggested a new model for the universe might be needed and sparked concerns that one of the sets of measurements might be incorrect.

"Now we've come up with an answer where Planck and ACT agree," said Simone Aiola, a researcher at the Flatiron Institute's Center for Computational Astrophysics and first author of one of two papers. "It speaks to the fact that these difficult measurements are reliable."


The oldest light in the universe

The Cosmic Microwave Background, leftover light from the big bang, carries a wealth of information about the universe&mdashfor those who can read it.

Fifty years ago, two radio astronomers from Bell Labs discovered a faint, ever-present hum in their telescope that they couldn&rsquot identify. After ruling out radio broadcasts, radar signals, a too-warm receiver and even droppings from pigeons nesting inside the scope, they realized they&rsquod found a soft cosmic static that originated from beyond our galaxy. Indeed, it seemed to fill all of space.

Fast-forward five decades, and the static has a well-known name: the cosmic microwave background, or CMB. Far from a featureless hum, these faint, cold photons, barely energetic enough to boost a thermometer above absolute zero, have been identified as the afterglow of the big bang.

This light&mdashthe oldest ever observed&mdashoffers a baby picture of the very early universe. How early? The most recent result, announced on Saint Patrick&rsquos Day 2014 by the researchers of the BICEP2 experiment, used extremely faint signals imprinted on CMB photons to reach back to the first trillionth of a trillionth of a trillionth of a second after the big bang&mdashalmost more of a cosmic sonogram than a baby picture. This image offered the first direct evidence for the era of cosmic inflation, when space itself ballooned outward in a turbocharged period of expansion.

CMB photons have more to tell us. Combined with theoretical models of cosmic growth and evolution, ongoing studies will expand this view of the very early universe while also looking forward in time. The goal is to create an entire album chronicling the growth of the universe from the very moments of its birth to today.

Further studies promise clear insight into which of the many different models of inflation shaped our universe, and can also help us understand dark matter, dark energy and the mass of the neutrino&mdashif researchers can read the CMB in enough detail.

That&rsquos not easy, though, because the afterglow has faded. During its epic 13-billion-year-plus journey, light that originally blazed through the universe has stretched with space itself, its waves growing billions of times longer and cooler and quieter.

Relic radiation

The Standard Model of Cosmology says that about 13.8 billion years ago, the universe was born from an unimaginably hot, dense state. Before a single second had ticked away, cosmic inflation increased the volume of the universe by an amount that varies according to the particular model, but always features a 10 followed by about 30 to 80 zeroes.

When inflation hit the brakes, leftover energy from that expansion created many of the particles we see around us today: gluons, quarks, photons, electrons and their bigger brethren, muons and taus, and neutrinos. Primordial photons scattered off free-floating electrons, bouncing around inside the gas cloud that was the universe. Hundreds of thousands of years later, the cosmic cloud of particles cooled enough that single protons and helium nuclei could capture the electrons they needed to form neutral hydrogen and helium. This rounded up the free electrons, clearing the fog and releasing the photons. The universe began to shine.

These photons are the cosmic microwave background. Although now weak, they are everywhere CMB photons bathe the Earth&mdashand every other star, planet, black hole and hunk of rock in the universe&mdashin their cold light.

Cosmic sonograms

The latest big discovery coaxed from CMB data peeks back into the earliest moments of the universe.

Using cutting-edge sensors, the BICEP2 telescope located at the South Pole detected a type of signal that has been predicted at one strength or another by every version of inflation theory out there: a type of polarization to the CMB light called &ldquoB-mode polarization.&rdquo

According to the theories, tiny variations in the energy of the pre-inflation universe caused primordial gravitational waves&mdashripples in the fabric of space-time&mdashthat ballooned outward with inflation. Even before they became the CMB, photons interacted with these ripples, causing the photons&rsquo wavelengths to take on a slight twist. It was this twist that the BICEP2 collaboration measured as a swirling polarization pattern.

&ldquoBICEP2 clearly detected B-mode polarization at precisely the angular scales predicted by inflation,&rdquo says Chao-Lin Kuo, one of four principal investigators on the experiment and a scientist at Stanford University and SLAC National Accelerator Laboratory. &ldquoThis is an incredible combination of big theoretical ideas, teamwork, focus and cutting-edge technologies. The development of mass-produced superconducting polarization detectors and quantum current sensors made a real difference to our success in getting to B-modes first.&rdquo

A discovery of this magnitude calls for further confirmation&mdashnot of the signals, which were very clear, but of their inflationary origin. If it holds, the B-mode polarization signals will also give scientists more details about the inflationary event that took place. For example, it can tell us about the energy scale of the universe&mdashessentially the amount of energy poured into the instant of inflation. The BICEP2 result puts this at about 10 16 billion electronvolts. For comparison, the Large Hadron Collider&rsquos most powerful proton beams smash together at 10 4 billion electronvolts&mdasha number with 12 fewer zeros than the first.

Such information can help scientists determine which of the many different models of inflation actually describes the beginning of our universe. To Walt Ogburn, a postdoctoral researcher at Stanford University and a member of the BICEP2 team, the first view of primordial B-mode polarization does more than turn inflationary theory into fact: It breaks through into uncharted territory in high-energy physics. &ldquoWhat drove inflation is not in the Standard Model,&rdquo Ogburn says. By definition, proof of inflation offers evidence that there&rsquos something more out there that&rsquos not yet discovered, and that something big we don&rsquot yet fully understand helped drive the evolution of the early universe.

Baby picture

The detection of B-mode polarization is the latest in a long string of scientific discoveries base on information coaxed from these scarce, faint photons.

The first successes in probing the CMB came almost two decades after it was identified. Beginning with Relikt-1, a Soviet satellite-based experiment launched in 1983, and continuing all the way up to the present, a variety of balloons and satellites have mapped the temperature of the CMB. They found it was 2.7 kelvin across the whole of the sky, with only small, scattered variations in temperature of about one part in 100,000.

In that temperature map cosmologists saw the image of the infant universe.

&ldquoWe&rsquove learned an enormous amount from the temperature [patterns],&rdquo says Lyman Page, also a cosmologist at Princeton. Page was one of the original researchers on what, until this year, was probably the best-known CMB instrument, the Wilkinson Microwave Anisotropy Probe. He now focuses on the Atacama Cosmology Telescope, and few people know more about how to make the CMB give up its secrets.

Page explains that both the overall sameness of the temperature and the pattern of these minor variations told cosmologists that when the universe began, it was compact enough to be in thermal equilibrium: a dense, nearly featureless plasma of immense energies. But within that plasma, quantum fluctuations caused tiny variations in energy density.

A section of the microwave sky mapped by the Wilkinson Microwave Anisotropy Probe. The colors correspond to slight temperature differences that provided the template for the formation of large-scale structure in the universe.

Then, during cosmic inflation, space grew enormously in all directions. This magnified the variations like an inflating balloon expands ink spots sprayed on it into larger and larger blotches.

This is the same process that generated the gravitational waves imaged by BICEP2. The gravitational waves left telltale swirling polarization patterns in the CMB without doing much else. However, the dense areas&mdash&ldquoblotches&rdquo on the otherwise smooth map of the sky&mdashbecame important seeds of all structures in the universe.

They grew and cooled, morphed from variations in energy density to variations in matter density. The denser regions attracted more matter as the universe continued to expand, eventually building up large-scale structures we see stitched across the universe today.

When combined with other theories and measurements, Page says, the temperature variations provide strong evidence that our universe began with the big bang. They have also helped cosmologists improve estimates for how much dark matter and dark energy existed in the early universe (and likely still exist today), and backed the notion that the geometry of the universe is flat.

&ldquoThe CMB is really a beautiful signal,&rdquo says the University of Chicago&rsquos John Carlstrom, who, like Page, is an expert in extracting information from a few faint photons. He leads the South Pole Telescope project, which uses several instruments mounted on a telescope not too far from BICEP2, to learn more about the CMB. The signal, he continues, offers &ldquovery precise measurements of conditions at recombination,&rdquo which is the name given to the time when the CMB photons escaped from the primordial cloud of cooling plasma.

These temperature maps&mdashin combination with the primordial B-mode signals detected by BICEP2&mdashcover a time period from a tiny fraction of a second after the birth of the universe to about 380,000 years after that. In the coming years, cosmologists want to expand that picture to include everything that&rsquos happened in the more than 13 billion years since recombination. Many predictions exist for what happened during this huge span of time, but scientists need rock-solid empirical data to compare their theoretical models against.

BICEP2 revealed a faint but distinctive twist in the polarization pattern of the CMB. Here the lines represent polarization the red and blue shading show the degree of the clockwise and counter-clockwise twist.

Filling in the photo album

CMB photons have more important informationto offer, and a new generation of experiments is listening to what they have to say. Situated mostly on the high, dry, cold deserts of the South Pole and the Atacama Plateau in Chile, or in high-flying balloons that rise above much of the atmosphere, new instruments use the CMB to refine our knowledge of how the universe has evolved.

As the CMB photons traveled through the universe, they were pulled this way and that by gravity, bearing witness to everything that happened on their way from the beginning to now. Using these photons as messengers, the new instruments are helping scientists carefully tease out the story of what the photons saw along their journey.

Interactions with the hot gas that surrounded and infused galaxy clusters, for example, left a mark on some of the photons in the form of a tiny boost in energy, which is detectable as a very slight adjustment to the temperature map.

The new instruments also measure a different type of B-mode polarization, added to the CMB photons long after inflation. This type of twist occurs when the photons brush up against the gravity of large-scale cosmic structures comprising both regular matter and dark matter, and it was detected for the first time just last year by SPTpol, a polarization-sensitive microwave camera mounted on the South Pole Telescope.

Taken together, these measurements of tiny temperature differences and polarization can help scientists map matter distributions over time and improve estimates of how much of the universe is made up of dark matter versus the normal matter we see in stars and planets. It can also help tease apart the difference between expansion due to the momentum left from the big bang and expansion due to dark energy. This will yield an accurate four-dimensional map of the universe, revealing the movement of matter through space and time.

Further measurements are poised to reveal more information about the contributions to our cosmos of a tiny particle with big implications: the neutrino. Its mass is currently not known to any respectable precision, yet this number is of great importance to predictions regarding the neutrinos&rsquo influence on the growing universe.

Experiments so far have seen three types of neutrinos, yet some theories predict a fourth type, called a sterile neutrino, as well.

&ldquoNeutrinos are the second most plentiful particle in the universe&mdashafter photons,&rdquo says Bradford Benson, a scientist at Fermilab and a member of the SPTpol team. &ldquoThe total mass of all the neutrinos in the universe should at least equal the mass of all the stars.&rdquo

When the universe was smaller, that neutrino mass could have had a significant influence on the universe&rsquos developing structure. As the universe expanded, two things happened: Clumps of heavier, slower-moving particles grew even bigger by pulling in more matter, while the light, speedy neutrinos escaped and space expanded while the number of neutrinos stayed the same such that, as their density decreased, their gravitational influence decreased as well.

As they traveled among the growing cosmic structures, the CMB photons recorded these changes in the relative density of neutrinos. Scientists are now mining this record to determine how the influence of neutrinos has evolved over time, and can use the information to estimate their mass. Combined with CMB measurements of dark matter and expansion due to dark energy, scientists expect this research to refine their view of the universe past and present, revealing how matter and energy interacted in the early universe to make the universe we see today.

Old light, new science

Using the CMB to discover primordial gravitational waves has been a tremendous step forward. &ldquoWhat&rsquos truly amazing is that the CMB may still hide more secrets even after we found the holy grail,&rdquo says Kuo, referring to BICEP2&rsquos discovery.

Temperature maps, scattered photons and twisted light still have more to tell us. Over the next decade, CMB measurements are poised to help us understand the immense forces of the big bang, illuminate the physics of the early universe and explain the matter and energy we see around us today.

&ldquoHaving this signal has helped turn cosmology into a precision science,&rdquo Carlstrom says. &ldquoWe&rsquove gone from being told, &lsquoYou guys don&rsquot really know what you&rsquore measuring&rsquo to having independent measurements with levels of precision that rival particle physics.&rdquo

And the benefits are only set to increase. &ldquoThe study of the CMB is a fantastic field, a very rich field,&rdquo Page says. &ldquoThe microwave background is still going to be a useful tool in 20 years.&rdquo


Contents

Beginning Edit

Before the advent of telescopes, astronomy was limited solely to unaided eyesight. Humans have been gazing at stars and other objects in the night sky for thousands of years, as is evident in the naming of many constellations, notably the largely Greek names used today.

Hans Lippershey, a German-Dutch spectacle maker, is commonly credited as being the first to invent the optical telescope. Lippershey is the first recorded person to apply for a patent for a telescope [1] however, it is unclear if Lippershey was the first to build a telescope. Based only on uncertain descriptions of the telescope for which Lippershey tried to obtain a patent, Galileo Galilei made a telescope with about 3× magnification in the following year. Galileo later made improved versions with up to 30× magnification. [ citation needed ] With a Galilean telescope, the observer could see magnified, upright images on Earth it was what is commonly known as a terrestrial telescope or a spyglass. Galileo could also use it to observe the sky, and for a time was one of those who could construct telescopes good enough for that purpose. On 25 August 1609, Galileo demonstrated one of his early telescopes, with a magnification of up to 8 or 9, to Venetian lawmakers. Galileo's telescopes were also a profitable sideline, selling them to merchants who found them useful both at sea and as items of trade. He published his initial telescopic astronomical observations in March 1610 in a brief treatise titled Sidereus Nuncius (Starry Messenger). [2]

Modern day Edit

In the modern day, visible-light astronomy is still practiced by many amateur astronomers, especially since telescopes are much more widely available for the public, as compared to when they were first being invented. Government agencies, such as NASA, are very involved in the modern day research and observation of visible objects and celestial bodies. In the modern day, the highest quality pictures and data are obtained via space telescopes telescopes that are outside of the Earth's atmosphere. This allows for much clearer observations, as the atmosphere is not disrupting the image and viewing quality of the telescope, meaning objects can be observed in much greater detail, and much more distant or low-light objects may be observed. Additionally, this means that observations are able to be made at any time, rather than only during the night.

Hubble Space Telescope Edit

The Hubble Space Telescope is a space telescope created by NASA, and was launched into low Earth orbit in 1990. [3] It is still in operation today. The Hubble Space Telescope's four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubble's images are some of the most detailed images ever taken, leading to many breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.

James Webb Space Telescope Edit

The James Webb Space Telescope is the formal successor of the Hubble Space Telescope. [4] It is set to launch on March 30, 2021, [5] and is "one of the most ambitious and technically complex missions NASA has ever set its focus upon." [6] The James Webb Space Telescope is a space-based telescope, and is set to orbit near the second Lagrange point of the Earth-Sun system, 1,500,000 km (930,000 mi) from Earth. [7]

There are three main types of telescopes used in visible-light astronomy:

    , which use lenses to form the image. Commonly used by amateur astronomers, especially for viewing brighter objects such as the Moon, and planets, due to lower cost and ease of usage. , which use mirrors to form the image. Commonly used for scientific purposes. , which use a combination of lenses and mirrors to form the image essentially a combination of refracting and reflecting telescopes.

Each type of telescope suffers from different types of aberration refracting telescopes have chromatic aberration, which causes colors to be shown on edges separating light and dark parts of the image, where there should not be such colors. This is due to the lens being unable to focus all colors to the same convergence point. [8] Reflecting telescopes suffer from several types of optical inaccuracies, such as off-axis aberrations near the edges of the field of view. Catadioptric telescopes vary in the types of optical inaccuracies present, as there are numerous catadioptric telescope designs.

The visibility of celestial objects in the night sky is affected by light pollution, with the presence of the Moon in the night sky historically hindering astronomical observation by increasing the amount of ambient lighting. With the advent of artificial light sources, however, light pollution has been a growing problem for viewing the night sky. Special filters and modifications to light fixtures can help to alleviate this problem, but for the best views, both professional and amateur optical astronomers seek viewing sites located far from major urban areas. In order to avoid light pollution of Earth's sky, among other reasons, many telescopes are put outside of the Earth's atmosphere, where not only light pollution, but also atmospheric distortion and obscuration are minimized.

The most commonly observed objects tend to be ones that do not require a telescope to view, such as the Moon, meteors, planets, constellations, and stars.

The Moon is a very commonly observed astronomical object, especially by amateur astronomers and skygazers. This is due to several reasons: the Moon is the brightest object in the night sky, the Moon is the largest object in the night sky, and the Moon has long been significant in many cultures, such as being the basis for many calendars. The Moon also does not require any kind of telescope or binoculars to see effectively, making it extremely convenient and common for people to observe. [ original research? ]

Meteors, often called "shooting stars" are also commonly observed. Meteor showers, such as the Perseids and Leonids, make viewing meteors much easier, as a multitude of meteors are visible in a relatively short period of time.

Planets are usually observed with the aid of a telescope or binoculars. Venus is likely the easiest planet to observe without the aid of any instruments, as it is very bright, and can even be seen in daylight. [9] However, Mars, Jupiter, and Saturn can also be seen without the aid of telescopes or binoculars.

Constellations and stars are also often observed, and have been used in the past for navigation, especially by ships at sea. [10] One of the most recognizable constellations is the Big Dipper, which is part of the constellation Ursa Major. Constellations also serve to help describe the location of other objects in the sky.


Taking a closer look at the age of the universe

For Bond, the similarities between the age of the universe and that of this old nearby star — both of which have been determined by different methods of analysis — is "an amazing scientific achievement which provides very strong evidence for the Big Bang picture of the universe". He said the problem with the age of the oldest stars is far less severe than it was in the 1990s when the stellar ages were approaching 18 billion years or, in one case, 20 billion years. "With the uncertainties of the determinations, the ages are now agreeing," Bond said.

Yet Matthews believes the problem has not yet been resolved. Astronomers at an international conference of top cosmologists at the Kavli Institute for Theoretical Physics in Santa Barbara, California, in July 2019 were puzzling over studies that suggested different ages for the universe. They were looking at measurements of galaxies that are relatively nearby which suggest the universe is younger by hundreds of millions of years compared to the age determined by the cosmic microwave background.

In fact, far from being 13.8 billion years old, as estimated by the European Planck space telescope's detailed measurements of cosmic radiation in 2013, the universe may be as young as 11.4 billion years. One of those behind the studies is Nobel laureate Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland.

The conclusions are based on the idea of an expanding universe, as shown in 1929 by Edwin Hubble. This is fundamental to the Big Bang — the understanding that there was once a state of hot denseness that exploded out, stretching space. It indicates a starting point that should be measurable, but fresh findings are suggesting that the expansion rate is actually around 10% higher than the one suggested by Planck.

Indeed, the Planck team determined that the expansion rate was 67.4 km per second per megaparsec, but more recent measurements taken of the expansion rate of the universe point to values of 73 or 74. That means there is a difference between the measurement of how fast the universe is expanding today and the predictions of how fast it should be expanding based on the physics of the early universe, Riess said. It's leading to a reassessment of accepted theories while also showing there is still much to learn about dark matter and dark energy, which are thought to be behind this conundrum.

A higher value for the Hubble Constant indicates a shorter age for the universe. A constant of 67.74 km per second per megaparsec would lead to an age of 13.8 billion years, whereas one of 73, or even as high as 77 as some studies have shown, would indicate a universe age no greater than 12.7 billion years. It's a mismatch that suggests, once again, that HD 140283 is older than the universe. It has also since been superseded by a 2019 study published in the journal Science that proposed a Hubble Constant of 82.4 — suggesting that the universe's age is only 11.4 billion years.

Matthews believes the answers lie in greater cosmological refinement. "I suspect that the observational cosmologists have missed something that creates this paradox, rather than the stellar astrophysicists," he said, pointing to the measurements of the stars being perhaps more accurate. "That's not because the cosmologists are in any way sloppier, but because age determination of the universe is subject to more and arguably trickier observational and theoretical uncertainties than that of stars."


Ancient Constellations

The Babylonians didn’t only draw pictures of the sky, either. They etched them into rock. By 3,200 years ago, they had carved the first known catalog of stars into stone tablets.

Yet, the titles given to some of those stars seem to have even older origins, apparently coming from the Sumerian people. This implies that formal knowledge of the stars stretches back to before recorded history.

These developments weren’t unique to the West, either. Similar histories played out on different timelines in varied cultures across the world. And that’s why many historians consider astronomy to be the oldest science.



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