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| The Milky Way Galaxy with our Sun and some other features labeled. There are 200 billion stars within 50,000 light years of Earth. |
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.
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| This image depicts the extent of the Milky Way galaxy -- a spiral galaxy of at least 200 billion stars. Our Sun is buried deep within the Orion Arm, about 26,000 light years from the center of the galaxy. Toward the center, stars are packed together much more closely than where we live. The Sagittarius Dwarf Galaxy is being slowly swallowed by the Milky Way. |
THE VISIBLE UNIVERSE
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 (186,000 miles per second). That's top speed in this universe—nothing can go faster—but it is relatively slow compared to the distances to nearby galaxies. 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 to 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. |
| Zooming out only 500,000 light years from the center of the Milky Way we see that we are surrounded by several dwarf galaxies each containing tens of millions of stars. Within 500,000 light years of Earth there are at least 225 billion stars. |
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| Zooming out 5 million light years from Earth there are 46 dwarf galaxies and over 700 billion stars. |
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.
COSMIC ACCELERATION
THE MULTIVERSE
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| 100 million light years from Earth. At this distance the Universe includes 2,500 large galaxies, 50,000 dwarf galaxies, and 200 trillion stars. |
COSMIC ACCELERATION
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| The Milky Way as seen from a highway in Gila Bend, Arizona, USA. |
Inflation implies a vastly expanded concept of what the universe is. But the concept is also helping scientists 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 also accelerating outward.
The discovery of cosmic acceleration was made by examining the light of supernovae. Astronomers believe they know the intrinsic brightness of a particular kind of supernovae, called "Type Ia," so they can calculate how far such an object must be from Earth by its apparent, or measured, brightness. They 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.
What astronomers look for in this combination of redshift and distance is the "growth rate" of the universe going back in time. This growth rate tells them about the gravity of all the matter in the universe—if there is a lot of matter it will slow down the growth rate over time.
MICROWAVE MESSAGES
Microwave radiation comes to us from the time in the past when the universe was a primordial fireball. We see a "surface" like we see the "surface" of the sun. We can't look into the sun (or a cloud in the sky) because of scattering of light. As with the sun and its spots, the surface of last scattering of the primordial fireball had structure caused by localized regions that were hotter or cooler, less or more dense.
The most pronounced of these structures at the cosmological surface of last scattering were governed by the distance that acoustic (pressure) waves could travel in the age of the universe back then, when the universe was about a half million years old. The size of these irregularities gives us a ruler. The radiation was emitted so long ago, so far away, that it has been redshifted down to millimeter wavelengths. So now millimeter experiments determine the angular size of the clumps caused by acoustic oscillations in the cooling universe at the surface of the last scattering.
We know how big the clumps were—a couple hundred thousand light years across. The relation between their real size, their distance, and the angular size that we observe is governed by the geometry of the universe. A universe dense with matter will distort the final size one way, an empty or almost empty universe will distort another way, and the flat universe of the inflation model will produce yet a different image, which we would intuitively call undistorted. The current results are in agreement with the flat universe of inflation.
This is not the full story. The theory of inflation predicts a precise recipe of how structure would form from little things merging into big things and tells us how many little things there should be for each big thing. The observations match with expectations if the mix of energy and matter is the same as that suggested by the supernovae experiments. Inflation also solves the old controversy over the Hubble Constant, the relationship between the rate galaxies are flying apart and the distances between them. If the Hubble Constant is large then galaxies are relatively close together and the implied age of the universe is way too short if the universe has been briskly expanding.
The universe cannot be younger than things in it. However, if the universe has been loitering and is now accelerating, then it is old enough and a large Hubble Constant is still possible. And we can actually make a direct measurement of the mass density of the universe by looking at the motions of galaxies that slosh in the gravitational wells of the matter. We find something that has come to be called "dark matter" there. If the universe is "flat," then this state is achieved through the sum of the mass and energy density. Measurements of gravity perturbations reveal just the needed complement of matter offsetting the repulsive energy indicated by the supernova measurements.
Microwave radiation comes to us from the time in the past when the universe was a primordial fireball. We see a "surface" like we see the "surface" of the sun. We can't look into the sun (or a cloud in the sky) because of scattering of light. As with the sun and its spots, the surface of last scattering of the primordial fireball had structure caused by localized regions that were hotter or cooler, less or more dense.
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| The Milky Way with lotus. |
The most pronounced of these structures at the cosmological surface of last scattering were governed by the distance that acoustic (pressure) waves could travel in the age of the universe back then, when the universe was about a half million years old. The size of these irregularities gives us a ruler. The radiation was emitted so long ago, so far away, that it has been redshifted down to millimeter wavelengths. So now millimeter experiments determine the angular size of the clumps caused by acoustic oscillations in the cooling universe at the surface of the last scattering.
We know how big the clumps were—a couple hundred thousand light years across. The relation between their real size, their distance, and the angular size that we observe is governed by the geometry of the universe. A universe dense with matter will distort the final size one way, an empty or almost empty universe will distort another way, and the flat universe of the inflation model will produce yet a different image, which we would intuitively call undistorted. The current results are in agreement with the flat universe of inflation.
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| The Hubble Constant |
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| The Milky Way as seen from Hawaii Volcanoes National Park on the Big Island of Hawaii. |
DARK MYSTERIES
The last couple of years have produced a remarkable convergence of evidence, all suggesting that we live in a universe with a few percent of the normal matter of our everyday experience, maybe 25% of something called "dark matter," which is a name given to hide our ignorance of what it is, and 75% of this energy that wants to push space apart—call it "dark energy." If true, then relatively recently in the history of the universe the "dark energy" has become dominant over "dark matter." During the transient dominance of dark matter, it caused the collapse into all the structure of the universe that we have come to know and appreciate.
Dark energy captivates physicists because it might provide a laboratory for the moment of creation. It may be that the present source of repulsion is quite different from the primordial situation. Certainly the energy density levels and time scales are vastly different. However, if we can understand the mechanism of the present acceleration perhaps we can get a clue about the acceleration at the first instant of our time.
So how big is the universe in the inflation model? It begs the question of what is going on at the boundaries and whether information could be communicated across universes. We suppose not. It may well be that only a tiny part of even our own universe is in our horizon, within the domain that we might hope to know.
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