How Large is the Universe?
The Totality of Everything that Exists

How Large is the Universe?
The Totality of Everything that Exists

The universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Observations of earlier stages in the development of the universe, which can be seen at great distances, suggest that the universe has been governed by the same physical laws and constants throughout most of its extent and history.

The universe has long captivated us with its immense scales of distance and time. How far does it stretch?

Where does it end and what lies beyond its star fields and streams of galaxies extending as far as telescopes can see?

These questions are beginning to yield to a series of extraordinary new lines of investigation and technologies that are letting us to peer into the most distant realms of the cosmos.

But also at the behavior of matter and energy on the smallest of scales.

The mind-blowing answer comes from a theory describing the birth of the universe in the first instant of time.

The universe is very large and possibly infinite in volume. The region visible from Earth (the observable universe) is about 92 billion light years across, based on where the expansion of space has taken the most distant objects observed.

For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years.

As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away.

There are probably more than 100 billion galaxies in the observable universe.

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.

But is believed that there are multiple universes that exist and that the universe is most likely a sphere.

Future NASA missions will continue to search for clues to the ultimate size and scale of our cosmic home.
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.


Dangerous Knowledge

In this one-off documentary, David Malone looks at four brilliant mathematicians - Georg Cantor, Ludwig Boltzmann, Kurt Gödel and Alan Turing - whose genius has profoundly affected us, but which tragically drove them insane and eventually led to them all committing suicide.

The film begins with Georg Cantor, the great mathematician whose work proved to be the foundation for much of the 20th-century mathematics.

He believed he was God's messenger and was eventually driven insane trying to prove his theories of infinity.

Ludwig Boltzmann's struggle to prove the existence of atoms and probability eventually drove him to suicide. Kurt Gödel, the introverted confidant of Einstein, proved that there would always be problems which were outside human logic.

His life ended in a sanatorium where he starved himself to death.

Finally, Alan Turing, the great Bletchley Park code breaker, father of computer science and homosexual, died trying to prove that some things are fundamentally unprovable. The film also talks to the latest in the line of thinkers who have continued to pursue the question of whether there are things that mathematics and the human mind cannot know.

They include Greg Chaitin, mathematician at the IBM TJ Watson Research Center, New York, and Roger Penrose. Dangerous Knowledge tackles some of the profound questions about the true nature of reality that mathematical thinkers are still trying to answer today.

Cosmic Journeys: Is the Universe Infinite?

Explore the biggest question of all... in 1080p. Since ancient times, we've looked into the night skies and wondered: How far do the stars stretch out into space? And what's beyond them? 

In modern times, we built giant telescopes that have allowed us to cast our gaze deep into the universe.

Astronomers have been able to look back to near the time of its birth. They've reconstructed the course of cosmic history in astonishing detail.

From intensive computer modeling, and myriad close observations, they've uncovered important clues to its ongoing evolution.

Many now conclude that what we can see, the stars and galaxies that stretch out to the limits of our vision, represent only a small fraction of all there is. 

Does the universe go on forever? Where do we fit within it? And how would the great thinkers have wrapped their brains around the far-out ideas on today's cutting edge? 

For those who find infinity hard to grasp, even troubling, you're not alone.
It's a concept that has long tormented even the best minds.

Over two thousand years ago, the Greek mathematician Pythagoras and his followers saw numerical relationships as the key to understanding the world around them. But in their investigation of geometric shapes, they discovered that some important ratios could not be expressed in simple numbers.

Take the circumference of a circle to its diameter, called Pi. Computer scientists recently calculated Pi to 5 trillion digits, confirming what the Greeks learned: there are no repeating patterns and no ending in sight.

The discovery of the so-called irrational numbers like Pi was so disturbing, legend has it, that one member of the Pythagorian cult, Hippassus, was drowned at sea for divulging their existence. 

A century later, the philosopher Zeno brought infinity into the open with a series of paradoxes: situations that are true, but strongly counter-intuitive. In this modern update of one of Zeno's paradoxes, say you have arrived at an intersection. But you are only allowed to cross the street in increments of half the distance to the other side. So to cross this finite distance, you must take an infinite number of steps.

In math today, it's a given that you can subdivide any length an infinite number of times, or find an infinity of points along a line. What made the idea of infinity so troubling to the Greeks is that it clashed with their goal of using numbers to explain the workings of the real world.

To the philosopher Aristotle, a century after Zeno, infinity evoked the formless chaos from which the world was thought to have emerged: a primordial state with no natural laws or limits, devoid of all form and content.

But if the universe is finite, what would happen if a warrior traveled to the edge and tossed a spear?

Where would it go? 

It would not fly off on an infinite journey, Aristotle said. Rather, it would join the motion of the stars in a crystalline sphere that encircled the Earth. To preserve the idea of a limited universe, Aristotle would craft an historic distinction.

On the one hand, Aristotle pointed to the irrational numbers such as Pi. Each new calculation results in an additional digit, but the final, final number in the string can never be specified. So Aristotle called it "potentially" infinite. 

Then there's the "actually infinite," like the total number of points or subdivisions along a line. It's literally uncountable. Aristotle reserved the status of "actually infinite" for the so-called "prime mover" that created the world and is beyond our capacity to understand. This became the basis for what's called the Cosmological, or First Cause, argument for the existence of God.

See How Big the Universe Really Is

The universe is very large and possibly infinite in volume. The region visible from Earth (the observable universe) is a sphere with a radius of about 46 billion light years, based on where the expansion of space has taken the most distant objects observed.

Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting the galaxy's center of mass.

Thus, a very rough estimate from these numbers would suggest there are around one sextillion stars in the observable universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion.

The observable matter is spread uniformly (homogeneously) throughout the universe, when averaged over distances longer than 300 million light-years.

However, on smaller length-scales, matter is observed to form "clumps", i.e., to cluster hierarchically; many atoms are condensed into stars, most stars into galaxies, most galaxies into clusters, superclusters and, finally, the largest-scale structures such as the Great Wall of galaxies.

The observable matter of the universe is also spread isotropically, meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content.

It appears that many of properties of the universe have special values in the sense that a universe where these properties only differ slightly would not be able to support intelligent life. Not all scientists agree that this fine-tuning exists. In particular, it is not known under what conditions intelligent life could form and what form or shape that would take.

A relevant observation in this discussion is that existence of an observer to observe fine-tuning, requires that the universe supports intelligent life. As such the conditional probability of observing a universe that is fine-tuned to support intelligent life is 1. This observation is known as the anthropic principle and is particularly relevant if the creation of the universe was probabilistic or if multiple universes with a variety of properties exist.