The most widely accepted model for the beginning of the universe suggests that the entirety of space and time were created by the Big Bang, an event that took place roughly 14 billion years ago (now estimated at 13.8 billion years back, to be precise). Before it "banged" into existence, the theory postulates that the universe was a very tiny, very hot, dense point, not entirely unlike a singularity, from which the entirety of everything we see around us was created.
It's unclear exactly what caused this infinitely dense point to spawn an entire universe. but there was one clue left behind which astronomers are using to help explain how everything we see around us came to be. This relic of the big bang is known as the Cosmic Microwave Background Radiation (CMBR), and it was formed roughly 380,000 years after the big bang.
As the name suggests, this figment of the big bang is detectable in the microwave range of frequencies, and faintly, but nearly uniformly, pervades every inch of the universe, even to this very day. By studying it, we can see the conditions that existed when the universe was just very, very young.
In order to explain some features of the universe, such as its uniform temperature and flat shape, physicists devised the theory of cosmic inflation, which now forms part of the standard theory of cosmic evolution.
Cosmic inflation took place a fraction of a second after the big bang, and describes how the universe expanded faster than light, starting from the size of a proton, and it just kept growing exponentially in size. Eventually, the expansion slowed down, which allowed the universe to cool enough for the first stars to coalesce.
Did It Begin With a Bounce?
Of course, there are other theories that aim to explain how the universe came to be. One, known as the big bounce, puts forth the notion that the big bang was not the beginning of everything, as many have believed. In fact, this theory suggests that a previous version of the universe existed long before the "bang" and inflation. In this cosmological model, the universe is cyclical — meaning it cycles between growing and then contracting, or "crunching."
In this theory, the universe expands for a period of time, perhaps a trillion years, driven by an energy field currently attributed to dark energy. Eventually, this energy field thins out, and the cosmos starts to gently deflate.
At this point, the expansion of the universe comes to a halt, and immense gravitational forces cause the universe to contract back in on itself — drawing all matter back together until everything is once again condensed back into a single, hot, tiny point — mimicking the conditions that existed just prior to the big bang. This contraction recharges the energy field, leading to a bounce, and the cycle starts anew.
The advantages of the big bounce theory are that it takes dark energy into account. If we go back again to the big bang theory, it's hypothesized that everything was very "soupy" up until the universe was around 300,000 years old. Temperatures then decreased drastically, the universe began to form the very first protons and electrons, which then fused into heavier elements such as hydrogen and helium (both still make up a large portion of all visible matter in the universe). Then came stars, planets, galaxies, and superclusters. The problem is dark energy, which makes up roughly 70 percent of the universe, we don't know what it is, where it came from, or ultimately whether it starts or stops. But it appears to be causing the rate of expansion of our universe to accelerate over time, rather than slow down
Open, Closed, Flat, or What?
It stands to reason that the expansion, which is drawing the universe apart at speeds that have yet to be precisely determined, but may exceed the speed of light for galaxies on the edge of the observable universe, can not go on forever. That, however, depends on whether the geometry of the universe is open, closed, or flat. To summarize:
For a flat universe, NASA explains, "There is exactly enough mass to cause the expansion to stop, but only after an infinite amount of time. Thus, the universe has no bounds and will also expand forever, but with the rate of expansion gradually approaching zero after an infinite amount of time. This is termed a flat universe or a Euclidian universe (the usual geometry of non-curved surfaces that we learn in high school is called Euclidian geometry)."
It's believed that if the fabric of spacetime is open, the expansion of the universe will never stop. Rather, stars and galaxies will drift farther and farther apart, until the matter is so far apart that hydrogen and helium can no longer coalesce and collapse into stars. The universe will experience heat death, sometimes called the big freeze, as stars slowly burn out — this is predicted to be what will happen if spacetime has a negative curvature.
If the universe has positive curvature, NASA explains, "There is more than enough mass to stop the present expansion of the universe. The universe in this case is not infinite, but it has no end (just as the area on the surface of a sphere is not infinite but there is no point on the sphere that could be called the "end"). The expansion will eventually stop and turn into a contraction. Thus, at some point in the future, the galaxies will stop receding from each other and begin approaching each other as the universe collapses on itself. This is called a closed universe."
The big bounce and the big crunch models both rely on the universe being closed.
What Does That Mean?
Think of the crunch as the polar opposite of the big bang. The universe is reverting itself back into the hot, dense singularity from which the entirety of spacetime may have originally sprung. There's still a lot of debate about the shape of the universe and what it means for the ultimate fate of everything, but the big bounce and the big crunch have not been ruled out, as the mysterious nature of dark energy makes it impossible to know how this force will influence matter in the far, far future.
If the big crunch ultimately rings true, and it appears to be compatible with Einstein's theory of general relativity, all matter will be drawn together, even black holes, until it forms supergalaxies and then ultra massive black holes. Finally, matter will combine to form one mega massive black hole. The universe will shrink and grow hotter as matter from stars, planets, and galaxies recombine, which is the opposite of what we are seeing now.
In addition to the shape of the universe needing to be consistent with our theories, the density of the universe is also another contributing factor. For the big crunch to work, all known matter must be dense enough, thus with sufficient gravitational pull, to effectively reverse the expansion. Something called critical density must be met.
As cool as these theories are, they are not the only candidates for how the universe might end. Dark energy is thought to keep driving galaxies farther and farther apart, and there is as yet no evidence this expansion will ever stop. Therefore, a theory called the big freeze, or heat death, may be more astronomically correct.
It says that the expansion of the universe will continue until even the nearest stars and galaxies we can currently see are driven farther and farther apart, until the universe becomes too cool for the most basic of functions to occur, such as star formation. The entropy, which is a measurement of order vs disorder in a closed system, will also increase to its maximum value — meaning what heat remains will be distributed uniformly throughout the universe. With star formation coming to a halt, the remaining stars will burn out, and the universe will become dark, cold, and uninhabitable.
Maybe we should be hoping for a big crunch followed by a big bounce?