# How big is the universe?

Our best estimates put the observable universe at about 93 billion light-years across 8.8×1023 kilometers. The real size, however, is probably much greater.

## Observing the universe — parallax and beyond

Let’s start our foray into the size of the universe with a very simple experiment: place your palm in front of your eyes. Look at it and focus on its position. Then close one eye, look at the palm again, and then switch eyes. Your hand appears to slightly move sideways, because of the different position of your eyes — this is called parallax.

By knowing the distance between your eyes and seeing the apparent displacement of your hand, we can calculate the distance to your hand. Now, imagine that instead of your eyes, we have two telescopes out in space, and instead of your palm we have a very distant object, say a star. We know how far apart the two telescopes are so we can calculate the distance to the star through parallax.

Thanks to the Earth’s orbit (which we can calculate precisely), we have exactly that: the ability to observe the same thing from two different points (the same telescope, moved around by the Earth’s orbit). This approach is used routinely by astronomers to calculate the distance to celestial objects.

A simplified illustration of the parallax of an object against a distant background due to a perspective shift. When viewed from “Viewpoint A”, the object appears to be in front of the blue square. When the viewpoint is changed to “Viewpoint B”, the object appears to have moved in front of the red square. Image credits: Booyabazooka / Wikipedia.

However, after around 100 light years, the distance becomes simply too great and the parallax method starts to lose its efficiency. Still, through parallax, we know that the universe is at the very least 200 light years across (100 in both directions) — something which at one point, seemed inconceivably large.

The real size, however, goes far beyond that.

### The observable universe — and a standard candle

This is where things start to get really interesting (and tricky). Let’s think about the age of the universe for a moment.

When we look at something that’s 1 light year away, it took the light one year to get from that object to us, so we’re seeing it the way it was one year ago. In a way, we’re looking through time and seeing the past.

We’ve seen galaxies that are over billions of years old, so the size of the universe must be at least a few billion light years across.

To finesse things, we know the age of the universe, within a pretty good margin, to be 13.7-13.8 billion light years, and we know that from two crucial pieces of evidence.

The first one has to do with universal expansion. We know that the universe is expanding, and it’s expanding at an accelerating rate. Assuming that it’s expanding similarly in all parts of the universe (which most scientists agree), all the objects in the universe are moving apart from one another at a similar pace.

Let’s take the galaxies, as incredibly massive “objects”: we know that they’re moving apart, and by knowing their current speeds and distances, as well as the rate at which the universal expansion is accelerating, we can calculate how long it took them to reach their current position.

This method puts the age of the universe at around 14 billion years.

RS Puppis is one of the brightest known Cepheid variable stars in the Milky Way — which makes it one of the most important “standard candles”. Image credits: Hubble / NASA.

The second method relies on measuring the age of the oldest clusters we’ve been able to observe. This is not straightforward and makes extensive use of our knowledge of stellar formation, particularly a group of stars called “main sequence stars”, which are the most common type of stars.

We know that these stars change color in time, becoming redder as they age. By measuring their color and brightness, we can calculate their age — they are a “standard candle”, an object whose brightness we can calculate mathematically.

But for the very oldest stars, even this doesn’t really work, and this is where the work of Henrietta Swan Leavitt, an American astronomer, comes in. Back in 1908, Henrietta realized that there was a special class of stars called Cepheid variables.

These stars have highly reliable brightness and pulsations, which enables astronomers to calculate just how old these stars are. Using this method, the age of the universe was calculated to be 13.7 billion years.

The fact that the two methods come up with such close values is encouraging, and subsequent studies and models have confirmed and refined this range. Currently, scientists are confident (99.1% accuracy) that the age of the universe is 13.81 billion years — meaning we have another important milestone in our quest to figure out the size of the universe.

So we have a smaller “yardstick” to measure things in our cosmic neighborhood, and a larger one to measure things in the observable universe. What’s next?

### The size of the observable universe

We might think that the size of the observable universe is 13.7 billion light years in all directions, so 27.4 billion light-years across. Spoiler alert: that’s not true! That’s just what we can see now — during the time it took the light to travel to us, the universe has continued to expand. Keep in mind: space itself is increasing.

Visualization of the expansion of the Universe. Image credits: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb.

So how big has the observable universe become since its inception?

The best answer we have comes from something called redshift. When a source of light comes from very far away, its wavelength starts to shift towards the red side of the spectrum. This type of Doppler shift was a key indication that the size of the universe is increasing, and can help researchers estimate how much the universe has expanded.

Basically, if we were to find some really old photons and analyze their spectral shift, we’d have a good estimate of how old something is, and how far away it currently lies. The earliest photons we have come from the so-called cosmic microwave background (CMBR), faint cosmic background radiation filling all space which represents the earliest known electromagnetic radiation.

Some of our most accurate estimates of the CMBR come from the Wilkinson Microwave Anisotropy Probe (WMAP), which, along with other estimates, found that farthest observable photons come from 46.5 billion light-years away.

The Cosmic Microwave Background temperature fluctuations from the 7-year Wilkinson Microwave Anisotropy Probe data seen over the full sky as a celestial sphere. Image credits: NASA.

The comoving distance from Earth to the edge of the observable universe is about 46.5 billion light-years 14.26 (gigaparsecs or 4.40×1026 meters) in any direction. So, although the light itself might have only traveled for 13.8 billion years, the distance from us to the point it came from is, at present, 46 billion light years away.

This would make the diameter of the observable universe about 93 billion light-years (the equivalent of 28 billion parsecs), assuming that the Earth occupies a relatively central position in the universe.

It should be noted that at the current time, the proper and the comoving distance between the Earth and the edge of the observable universe are defined as equal (for the sake of simplicity). This is merely a convention — at other times, the scale factor was different than 1.

The same measurements described above concluded that at the time the CMBR was emitted, the proper distance was only 42 million light-years.

Another visualization of the universal expansion. Image credits: NASA, Goddard Space Flight Center.

So, to the best of our knowledge, the size of the observable universe is 93 billion light-years across. It is almost certainly bigger than that, but we don’t have any substantial evidence to judge its size outside of that.

However, one statistical estimate carried out by Oxford researchers found that the universe might be 251 times larger than the observable universe, which would put it at 23343 light-years across. That’s truly humbling, and some studies go even beyond that. Estimates for the total size of the universe, reach as high as  megaparsecs, as implied by one resolution of the No-Boundary Proposal. Just so you can get an idea of how big that number is, it doesn’t even matter what units of measure you express it in — whether it be nanometers or megaparsecs, the difference would simply get lost in the irrelevant final digits.

### Universal expansion

Think of the universe as a muffin dough. Think of matter inside this space as poppy seeds inside this dough. As the dough is baked, it expands, and the space between all poppy seeds increases — similarly, universal expansion drives matter apart, though the process is only detectable at cosmological scales.

## NASA – How Big is Our Universe?

The universe is a big, big place.

But how big? And how do we know?

Throughout history, humans have used a variety of techniques and methods to help them answer the questions 'How far?' and 'How big?' Generations of explorers have looked deeper and deeper into the vast expanse of the universe. And the journey continues today, as new methods are used, and new discoveries are made. (To learn more about distance, visit How Big is Our Universe.)

In the third century B.C., Aristarchus of Samos asked the question 'How far away is the Moon?' He was able to measure the distance by looking at the shadow of the Earth on the Moon during a lunar eclipse. It was Edmund Halley, famous for predicting the return of the comet that bears his name, who three centuries ago found a way to measure the distance to the Sun and to the planet Venus. He knew that the planet Venus would very rarely, every 121 years, pass directly between the Earth and the Sun. The apparent position of the planet, relative to the disk of the Sun behind it, is shifted depending on where you are on Earth. And how different that shift is depends on the distance from both Venus and the Sun to the Earth. This rare event, the transit of Venus, occurred again quite recently, June 8, 2004.

It was knowing this fundamental distance from the Earth to the Sun that helped us find the true scale of the entire Solar system for the first time.

Image to right: Our sun, the nearest star, is 93 million miles away. That's why the sun, which is a million times the size of the Earth, looks so small. It would take the Space Shuttle seven months to fly there. Credit: SOHO – ESA & NASA

When we leave the solar system, we find our star and its planets are just one small part of the Milky Way galaxy. The Milky Way is a huge city of stars, so big that even at the speed of light, it would take 100,000 years to travel across it. All the stars in the night sky, including our Sun, are just some of the residents of this galaxy, along with millions of other stars too faint to be seen. The further away a star is, the fainter it looks. Astronomers use this as a clue to figure out the distance to stars that are very far away. But how do you know if the star really is far away, or just not very bright to begin with? This problem was solved in 1908 when Henrietta Leavitt discovered a way to tell the 'wattage' of certain stars that changed their pulse rate linked to their wattage. This allowed their distances to be measured all the way across the Milky Way.

 Image above: How Big is the Milky Way? Imagine that our entire Solar System were the size of a quarter. The Sun is now a microscopic speck of dust, as are its nine planets, whose orbits are represented by the flat disc of the coin. How far away is the nearest star to our sun? In our model, Proxima Centauri (and any planets that might be around it) would be another quarter, two soccer fields away. This is the typical separation of stars in our part of the galaxy. Credit: Hubble Heritage Team (AURA/STSCI/NASA); US Mint

Beyond our own galaxy lies a vast expanse of galaxies. The deeper we see into space, the more galaxies we discover. There are billions of galaxies, the most distant of which are so far away that the light arriving from them on Earth today set out from the galaxies billions of years ago. So we see them not as they are today, but as they looked long before there was any life on Earth.

Finding the distance to these very distant galaxies is challenging, but astronomers can do so by watching for incredibly bright exploding stars called supernovae. Some types of exploding stars have a known brightness – wattage – so we can figure out how far they are by measuring how bright they appear to us, and therefore how far away it is to their home galaxy.

Image to right: The picture on the right was taken three weeks after the one on the left. In that time, a star at the edge of one of these distant galaxies has exploded — “gone supernova.” Can you spot the supernova in the picture at right? Even though the explosion is as bright as a billion suns, it is so far away that it is just a speck of light. Credit: NASA and J. Blakeslee (JHU)

The image below is both the oldest and youngest picture ever taken. It is the oldest because it has taken the light nearly 14 billion years to reach us. And it is the youngest because it is a snapshot of our newborn universe, long before the first stars and galaxies formed. The bright patterns show clumps of simple matter that will eventually form stars and galaxies. This is as far as we can see into the universe. It is time, not space, which limits our view. Beyond a certain distance, light hasn't had time to reach us yet.

 Image above: What is the furthest we can see? In 2003, NASA's WMAP satellite took images of the most distant part of the universe observable from Earth. The image shows the furthest we can see using any form of light. The patterns show clumps of matter that eventually formed into galaxies of stars. Credit: NASA/WMAP Science Team

So how big is the universe? No one knows if the universe is infinitely large, or even if ours is the only universe that exists. And other parts of the universe, very far away, might be quite different from the universe closer to home. Future NASA missions will continue to search for clues to the ultimate size and scale of our cosmic home. Go on the full exploration of the size of our universe at: How big is our universe? Beautiful images and straight-forward methods and ideas take you from our solar system, into the realm of the stars, the galaxies and finally into the vast panorama of the observable universe. You can also download and print a pdf version of these explorations.

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The Universe Forum talks more about size and distance in the universe.
For kids 14 and up, check out investigations into the destiny of our universe.
NASA's Astronomy Picture of the Day Web site contains images and links to exciting astronomical discoveries and observations. It also includes a searchable directory and glossary of astronomical phenomena.
Similar to Astronomy Picture of the Day, this site has weekly updates of news and images from telescopes observing in ultraviolet, X-Ray and gamma-ray light.Office of Space Science

## NOVA Online | Runaway Universe | How Big is the Universe?

 There are billions of galaxies in the visible universe.

How Big is the Universe?
by Brent Tully How big is the universe? Could it be infinitely large? If the universe has an
edge, what is beyond the edge? And if the universe had a beginning, what was
going on before that? Our experience of the everyday world does not prepare us to grasp the concept
of an infinite universe. And yet, trying to imagine that the cosmos actually
has a boundary does not make things any easier.

There is an edge to what we are able to see and could ever possibly see
in the universe. Light travels at 300,000 kilometers per second. That's top speed in this universe—nothing can go faster—but it's relatively slow compared to the distances to be traveled. The
nearest big galaxy to our Milky Way, the Andromeda galaxy, is two million
light-years away. The most distant galaxies we can now see are 10 or 12 billion
light-years away. We could never see a galaxy that is farther away in light
travel time than the universe is old—an estimated 14 billion or so years.
Thus, we are surrounded by a “horizon” that we cannot look beyond—a horizon
set by the distance that light can travel over the age of the universe.

This horizon describes the visible universe—a region some 28 billion
light years in diameter.

But what are the horizons of a civilization that
inhabits the most distant galaxies we see? And what about galaxies at the
limits of their vision? There is every reason to think that the universe
extends a long way beyond the part of the universe we can see. In fact, a
variety of observations suggest that our visible patch may be a small fraction—maybe an infinitely small fraction—of the whole universe.

This view of the universe fits with the currently popular idea that the
universe began with a vast expansion of size. The idea describes a kind of
undirected energy present in the vacuum of space, called scalar fields, that
somehow got channeled into a process called “inflation.” By conservative
estimates, the universe expanded so much during this period that something the
size of an atom inflated to the size of a galaxy. If this grand idea is correct, then the universe is larger than we ever could
have imagined. But the question remains: Is there a boundary, and if so, what
lies in the voids beyond? The answer, according to some cosmologists, is truly
mind-boggling. If the universe sprung forth in this manner, then probably
inflation has occurred in other places, perhaps an infinite number of places,
beyond our horizon and outside of our time. The implication is that there are
other universes, perhaps similar to ours or vastly different, each in its own
space and begun in its own time.

 The universe began with a vast explosion that generated space and time.

Inflation implies a vastly expanded concept of what the universe is. But the
concept is also helping us to understand the universe we see around us. Take,
for example, the recent observation that the universe is not only expanding—a fact astronomers have known for over seven decades—but actually
accelerating outward. That discovery is the subject of NOVA's program “Runaway
Universe.”

While we can never directly “see” the whole of the universe or glimpse its
farthest horizons, we can discover how it is behaving—how fast it's growing,
whether its growth will one day come to a halt, and what forces have been
driving its evolution on the largest of scales. The evidence for the cosmic
acceleration—the observations of distant exploding stars called supernovae
(see Birth of a Supernova)—provides a window onto these
behaviors.

The discovery of cosmic acceleration was made by examining the light of
supernovae. We astronomers believe we know the intrinsic brightness of a
particular kind of supernovae, called “Type Ia,” so we can calculate how far
such an object must be from us by its apparent, or measured, brightness.

We
also know how fast the supernovae—and the galaxies they're in—are rushing
away from us by measuring their “redshift.” Redshift refers to a color shift in
the light of galaxies toward the red end of the spectrum as they race away from
us. The faster a galaxy is moving away, the redder its light becomes.

(For more
on this phenomenon, go to Moving Targets.)

## How Big is the Universe?

Called the eXtreme Deep Field, or XDF, the photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon. Image released September 25, 2012.

(Image: © NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team)

As technology has evolved, astronomers are able to look back in time to the moments just after the Big Bang. This might seem to imply that the entire universe lies within our view. But the size of the universe depends on a number of things, including its shape and expansion. Just how big is the universe? The truth is, scientists can't put a number on it.

Related: What Is Big Bang Theory?

### The observable universe

In 2013, the European Space Agency's Planck space mission released the most accurate and detailed map ever map of the universe's oldest light. The map revealed that the universe is 13.8 billion years old. Planck calculated the age by studying the cosmic microwave background.

“The cosmic microwave background light is a traveler from far away and long ago,” Charles Lawrence, the U.S. project scientist for the mission at NASA's Jet Propulsion Laboratory in Pasadena, California, said in a statement. “When it arrives, it tells us about the whole history of our universe.”

## How Big Was The Universe At The Moment Of Its Creation?

An ultra-deep view of galaxies many billions of light years away in the distant Universe.

NASA, ESA, R. Windhorst, S. Cohen, and M. Mechtley (ASU), R. O’Connell (UVa), P. McCarthy (Carnegie Obs), N. Hathi (UC Riverside), R. Ryan (UC Davis), & H. Yan (tOSU)

You might think of the Universe as infinite, and quite honestly, it might truly be infinite, but we don't think we'll ever know for sure.

Thanks to the Big Bang — the fact that the Universe had a birthday, or that we can only go back a finite amount of time — and the fact that the speed of light is finite, we're limited in how much of the Universe we can see. By time you get to today, the observable Universe, at 13.8 billion years old, extends for 46.

1 billion light years in all directions from us. So how big was it all the way back then, some 13.8 billion years ago? Let's look to the Universe we see to find out.

The Hercules galaxy cluster showcases a great concentration of galaxies many hundreds of millions of… [+] light years away.

ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute

When we look out at the distant galaxies, as far as our telescopes can view, there are some things that are easy to measure, including:

• what its redshift is, or how much its light has shifted from an inertial frame-of-rest,
• how bright it appears to be, or how much light we can measure from the object at our great distance,
• and how big it appears to be, or how many angular degrees it takes up on the sky.

These are very important, because if we know what the speed of light is (one of the few things we know exactly), and how intrinsically either bright or big the object we're looking at is (which we think we know; more in a second), then we can use this information all together to know how far away any object actually is.

Standard candles (L) and standard rulers (R) are two different techniques astronomers use to measure… [+] the expansion of space at various times/distances in the past.

## How Big Is the Universe?

If you've ever dreamed of time traveling, just look out at the night sky; the glimmers you see are really snapshots of the distant past. That's because those stars, planets and galaxies are so far away that the light from even the closest ones can take tens of thousands of years to reach Earth.

The universe is undoubtedly a big place. But just how big is it?

“That may be something that we actually never know,” Sarah Gallagher, an astrophysicist at Western University in Ontario, Canada, told Live Science. The size of the universe is one of the fundamental questions of astrophysics. It also might be impossible to answer. But that doesn't stop scientists from trying.

Related: What Happens in Intergalactic Space?

The closer an object is in the universe, the easier its distance is to measure, Gallagher said. The sun? Piece of cake. The moon? Even easier. All scientists have to do is shine a beam of light upward and measure the amount of time it takes for that beam to bounce off the moon's surface and back down to Earth.

But the most distant objects in our galaxy are trickier, Gallagher said. After all, reaching them would take a very strong beam of light. And even if we had the technological capabilities to shine a light that far, who has thousands of years to wait around for the beam to bounce off the universe's distant exoplanets and return back to us?

## How Big Is the Universe of Exons? – PubMed

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If genes have been assembled from exon subunits, the frequency with which exons are reused leads to an estimate of the size of the underlying exon universe.

An exon database was constructed from available protein sequences, and homologous exons were identified on the basis of amino acid identity; statistically significant matches were determined by Monte Carlo methods.

It is estimated that only 1000 to 7000 exons were needed to construct all proteins.

### Comment in

• The universe of exons revisited. Saxonov S, Gilbert W. Saxonov S, et al. Genetica. 2003 Jul;118(2-3):267-78. Genetica. 2003. PMID: 12868615
• Counting and discounting the universe of exons. Doolittle RF. Doolittle RF. Science. 1991 Aug 9;253(5020):677-80. doi: 10.1126/science.1871603. Science. 1991. PMID: 1871603 No abstract available.