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Cosmology Theories
What is the universe?
In talking about the universe it’s important to establish some ground rules. There are two definitions of the universe. The first is the true universe. The word universe means “all that is”, incorporating all energy, matter, time and space. The second definition describes the observable universe. This is the universe we know, because we look out there and see it, or know by some indirect method that it is there.
Determining the evolution of the universe
Our predictions about the ultimate destiny of the universe depend on our observations of the distribution of matter in space. The amount of matter, and how it is distributed, is known as the critical density.
If the density of the universe is found to be lower than the critical density, then it will expand in an accelerating fashion. If the density is equal to the critical density, then the universe will expand forever but at a decreasing rate. Finally, if the density is found to be greater than the critical density, the universe will eventually stop expanding and then collapse.
Two variables that are important to making an accurate prediction of our ultimate fate are the amounts of dark matter and dark energy in the universe. Another factor is if the “energy” of dark energy changes over time (the so-called phantom energy factor). Recent studies from the Chandra X-Ray Observatory suggest that dark energy appears to be constant across space, with a strength that never changes with distance or time.
The evolution of the universe, then, is determined by the interplay of the rate of its expansion caused by the Big Bang, and the pull or push of gravity. Traditional gravity would attempt to counteract expansion, while dark energy, which acts as a sort of “gravity that pushes”, would aid expansion.
Possible shapes of the universe
The universe could have one of three shapes: negatively curved, positively curved, or flat. Each is described below, along with the observational evidence that would be used to determine it.
Open Universe: Galaxies increase in density with distance. The universe is shaped like a saddle or a potato chip, having a negative curvature. Space expands with a positive acceleration. The universe ends in “heat death”, with the Big Freeze, or the Big Rip.
Closed Universe: Galaxies decrease in density with distance. The universe is spherical, having a positive curvature. The rate of expansion slows with time. Ultimately gravity will reverse expansion and the universe will collapse upon itself, causing the Big Crunch.
Flat Universe: Galaxy distribution is uniform. The universe is flat, like a sheet, cylinder, Möbius strip or even a doughnut. There is no observable acceleration, but expansion is infinite. The universe will not end, but will eventually suffer heat death.
Where is the Center of the Universe?
There is no center. When creation happened, the universe began everywhere. Time also began then, so there was nothing “before” the universe because there was no “before”.
What and where is the edge of the universe?
Beyond Galaxies
As we look back to about 13.7 billion years ago we see the farthest galaxies fade away and there is a gap called “the dark era” where we see no more matter. At nearly 13.8 billion years ago radio telescopes can detect the relic of the Big Bang – the CMB - microwave radiation that has redshifted from the heat and light of the creation event. Its age is about 379,000 years after the Big Bang.
The CMB
The edge of the observable universe is marked by the cosmic microwave background. Even though the universe emitted its first light 13.8 billion years ago, thanks to the expansion of the universe, those first galaxies are now 46.5 billion light years distant. So the observable universe is about 93 billion light years across. Thus we should in theory find matter 46.5 billion light years from us in every direction, no matter where we live. (Remember, there is no center to the universe.) But can we really see galaxies 46.5 billion light years away? No, because we can only see galaxies that have had enough time to send their light to us.
How do we use the CMB to determine the age of the universe?
The cosmic microwave background is not perfectly smooth- it has tiny fluctuations that show the density of matter at “the edge” is not uniform. By studying these small ripples of energy and density, and by measuring the speed of the redshifted waves, astronomers have determined that the Big Bang happened approximately 13.8 billion years ago.
The Privilege of Time
We will never be able to see all of the universe because we do not have the full privilege of time. We can only see light up to a radius of 13.8 billion light-years in any direction, since that is when the universe began and light can only travel at the speed of light. But the matter that was created early on is now much farther away.
Here’s an example: A quasar with a redshift of Z = 6 would have taken about 12.7 billion years to send its light to us. That distance is measured in light years and (12.7 Billion LY), but that is where the quasar was when it emitted the light, not where it is now. The time it takes the light to travel to Earth is called the look-back time. But in that time, the quasar has traveled farther and is now at a distance of 27.billion light years. The “true” distance to the quasar is known as its luminosity distance.
So… How can the quasar travel faster than light?!
Good question! The quasar is not traveling faster than light. The rate of the expansion of the universe is moving faster than light. The quasar is just along for the ride.
The universe is expanding faster than light can travel towards us, so in the future, we will be able to see less and less of it. This gives rise to the adage: Be a cosmologist today, because in the future, you will not have anything to out there observe!
What’s beyond the edge?
Nothing. Nothing is defined as the absence of all space and time and matter and energy. The universe is “all that is”- it is self-contained and does not expand into anything else.
We cannot go past the edge of our universe because we are made of its “stuff” -matter and energy, space and time. We would simply be dragging the universe along with us.
Current Observations
The most current data support the Big Bang-Inflationary theory of the early formation of the universe. The universe is expanding at an accelerating rate. Its shape is close to flat and its density is nearly at 1.0, the critical density. The overall pattern to the distribution of matter in the universe is in “bubbles and voids”. Galaxy clusters appear to align in curved sheets as if on the surfaces of giant bubbles, with no observable matter inside (voids).
The universe is 13.79 billion years old. The observable universe spans an area 93 billion Light Years in diameter but the real universe is probably infinite in size. It will most likely undergo an ultimate heat death, also known as the Big Freeze.
End Theories for the Universe
The Big Freeze: The universe will continue to expand at a constant rate, and in time, all matter will eventually be so spread apart that it will no longer move or exchange energy with its surroundings. This ultimate state of the universe is known as the heat death.
The Big Crunch: If the universe has too little initial energy to continue its expansion, at some point it will collapse and the gravitational redshift force will pull the universe back to its starting condition, or perhaps into one gigantic black hole.
The Big Rip: The universe expands so quickly that it accelerates like a runaway train. Gravity loses its grip, and matter is shredded from its composite forms (molecules, atoms and nucleons).
The following is for those who prefer no ultimate demise:
The Big Bounce (Oscillating): The universe will continue to recycle itself in crunches and bangs, UNLESS in one of its attempts, the rate of inflation is too rapid or slow. If too rapid, it will die a “heat death”. If too slow, it will fail to inflate, perhaps remaining in a compactified “cosmic string” condition.
Multiverses, Branes and Strings
Could there be other “universes” that exist in different dimensions or realms of size? This model was first proposed as the “atom in a giant’s tea cup”. It is important to note that these are not theories, since there is no observational evidence to support them. They were derived from purely mathematical origins. There are two popular models of multiverses: Parallel universes that take the form of space-time foams or bubbles, and the membrane/string models.
Bubble or “many worlds” universe models
In these models there are many universes that exist, but we can only detect ours because we were made to exist in it. At any one instant, there are many universes simultaneously coexisting and evolving in various dimensions and versions of space-time, but all are undetectable to each other.
String Theory/ Branes
Branes (short for membranes) are iterations of the universe with a defined number of dimensions, up to a maximum of 9. According to this model, we live in a 4 dimensional brane where all other dimensions are “compactified”. The hidden dimensions are collectively called “the bulk”.
String theory permits up to 26 such dimensions, though most cosmologists who work on string theory predict only 9-11 dimensions. Strings are incredibly small (10-35m) vibrating entities that replace the traditional particles predicted by quantum physics. They vibrate in all dimensions simultaneously. According to these theories, strings (or looped versions, called superstrings) can stretch between adjacent branes. Gravity can also exist in multiple “braneworlds” simultaneously, but electromagnetic energy and matter are confined to our brane (or others like us, though the types of matter and energy in other branes can be totally different than the ones we know). You can think of branes as “parallel universes”.
What happens when branes touch each other? If they slowly drift together and touch, it could produce the end of the universe in the “Big Crunch”, since this will cause the collapse of one or more dimensions. But when they disconnect again, the universe is recreated with a new epoch of inflation. Some theorists suggest that this is in fact what happened after the Big Bang instead of inflation. It has been nicknamed “the Big Splat”.
Imagine the early universe a sort of space-time foam where a bubble pinches off and separates from it. This is like the separating of two branes.
Is your “brain” ready to explode yet??
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