The earliest phases of the universe we know of began with the Big Bang, in which the exploding universe was filled with energetic particles, antiparticles, and radiation. But in order to build such a universe, we need a period dominated by the inherent energy of the universe itself, expanding at an exponential rate and eventually decaying, resulting in a universe full of matter, antimatter, and radiation.
The deepest part of the distant universe shows galaxies being pushed away by dark energy. Can these dark energies be linked to the expansion phenomenon that triggers everything in the first place?
Now, 13.8 billion years after the expansion ended, the matter and radiation in the universe have become so sparse and so dense that a new ingredient has been discovered: dark energy. Dark energy is the energy inherent in space itself and is expanding the universe at an exponential rate. Although there are some differences between dark energy and expansion, there are also some unique similarities. Are these two phenomena likely to be related? If so, does this mean that the beginning (beginning) and end (end) of our universe are connected?
During expansion, at the quantum scale, fluctuations in space-time itself stretch the entire universe, leading to imperfections of density and gravitational waves.
It would be strange to us if there were two completely different forces or mechanisms that inflated the universe: a billion years ago and today. But, when it comes to the universe, there are a lot of things that seem strange to us. First of all, the universe is definitely expanding, but it doesn't require any kind of force. In fact, when you choose a universe like our own, it is:
Dominated by Einstein's general theory of relativity,
Full of matter, radiation and any other "stuff" you like,
On average, it is similar in all positions and in all directions (isotropic),
You come to an interesting and uncomfortable conclusion. Einstein himself first came to this conclusion in the early years of relativity itself: such a universe would not have been inherently unstable by gravitational collapse.
Over time, under the influence of gravity, an almost unified universe will form a web of cosmic structures.
In other words, unless you come up with some "magical solution" to the problem, your universe will have to expand or contract, and both solutions are possible. Unless you concoct some new type of force, you can't keep the universe still.
Of course, Edwin Hubble's work isn't done yet. In addition to not knowing that the universe is expanding, we don't even know that those spiral-shaped objects in the sky are objects in our own milky way or that they are another galaxy in their own right. Because Einstein was most fond of static universes at the time (like most people), he made a temporary fix to keep the universe static: he introduced the concept of the cosmic constant.
The Einstein field equation, whose cosmological constant is included as the last term on the left.
The central idea of Einstein's theory of relativity is that equations have two aspects: matter and energy, and space-time. It says that the existence of matter and energy determines the curvature and evolution of space-time, and the way space-time curves and evolves determines the fate of each of its material and energy quanta.
The addition of the cosmological constant goes something like this: "This new type of energy inherent in space itself causes the structure of the universe to expand at a constant rate." "So if there's gravity, then all matter and energy is collapsing the universe, and when you have this cosmological constant to expand the universe, you can get a static universe after all." All you need to do is make these two ratios match and cancel each other out exactly.
If the universe were completely unified, or if everything were perfectly distributed, then no large-scale structure would have been formed. But any tiny flaw can lead to clumps and voids, as the universe itself shows.
It turns out that the universe is expanding, and there is no need for a cosmic constant there to counteract gravity. Instead, there is an initial condition in which the universe begins to expand very rapidly, canceling out the gravitational pull from all matter and energy. The universe is not shrinking, it is expanding, and the rate of expansion is slowing down.
Now, there are two natural questions to ask, which are actually questions to be asked since the discoveries of the 1920s, after which:
What caused the universe to begin to expand at this rapid rate sooner rather than later?
What will be the fate of the universe? Will it ever expand, will it eventually reverse and collapse, will it be on the boundary between the two, or something else?
Illustration: The different fates of the universe. The actual, accelerated fate is shown in the figure on the right. The Big Bang itself did not explain the origin of the universe itself.
For a century and a half, the first question remained unanswered, although interestingly, Willem de Sitter immediately made the initial suggestion that it was a cosmological constant that led to the onset of this expansion.
Previously thought to come only from cosmological constants, Alan Guth's revelation at the end of 1979 led to the creation of the expansion of the universe as a method of "exploding" the universe.
Eventually, in the early 1980s, the theory of cosmic expansion emerged, proposing an early stage of exponential inflation in which the universe was dominated by something very similar to the cosmological constant.
Now, it can't be a true cosmic constant (also known as vacuum energy) because the universe won't remain in this state forever. Instead, the universe may be in a false vacuum state in which space itself has some inherent energy and then decays to a lower energy state, leading to the appearance of matter and radiation: the blazing Big Bang!
Note: Large structures form differently in the universe produced by inflation and its predictions (L) than in the cosmic string control network (R).
There are many other predictions about the expansion of the universe, and all but one of them has been confirmed, so we accept that an early stage in the universe does exist.
But when we turn to the second question, which is about the fate of the universe, we find something very strange. Although we had expected some sort of race between initial, rapid expansion and the effects of gravity on all matter and energy in the universe, we found that there was a new form of energy: something called dark energy. Don't you know? As far as we know, this dark energy seems to have the same form as the cosmological constant.
The distant fate of the universe offers many possibilities, but if dark energy is indeed a constant, as the data indicate, it will continue along the red curve. The distant fate of the universe offers many possibilities, but if dark energy is indeed a constant, it will continue along the red curve, as the data suggests.
Now, the exponential extensions of the two types (early and late) are very different in detail.
The inflationary periods of the early universe lasted for uncertain times — possibly as short as 10 ^ (-33) seconds , possibly as long as almost infinitely long , while dark energy today has dominated for 6 billion years.
The early state of expansion was unusually rapid, with the universe expanding about 10^50 times today. In contrast, today's dark energy accounts for about 70% of today's expansion rate.
The early states must have somehow coupled with matter and radiation. Assuming that quantum field theory is correct, at sufficiently high energies, there must be some sort of "inflaton" particle. Late dark energy has no known coupling at all.
That being said, there are some similarities.
Of the four possible fates of the universe, only the last one matches our observations.
They all have the same (or indistinguishable) equation of state, which means that the relationship between the cosmic scale and time of the two is the same.
They have the same relationship between the energy density and pressure they elicit in general relativity.
They all cause the same type of expansion in the universe — exponential expansion.
Note: The "open funnel" section of the illustration indicates exponential inflation, which occurs both at the beginning (during the inflation) and at the end (when dark energy dominates).
But do they have a relationship? It's hard to say. The reason, of course, is that we don't know much about any of them! When I think about swelling, I like to imagine a 2-liter half-soda soda bottle. I imagine a drop of oil floating on top of the liquid inside. This high-energy state is like the universe during expansion.
If the expansion is like starting from the "top" of a full soda bottle, then dark energy is like realizing that the bottom of your soda bottle is not completely empty. In both cases, space itself has an inherent energy. The inflation factor is much larger, but the effect of dark energy is non-zero.
Then something happens that causes the liquid to drain out of the bottle. Of course, the oil sinks to the bottom in a low-energy state.
However, if the rise of the droplet does not begin at the bottom— non-zero, but some finite non-zero value (e.g., when the Symmetry of the Higgs field ruptures) — this may be the cause of dark energy. The model that links these two fields (expansion field and dark energy field) is often referred to as the typical model.
It is very easy to build a typical model that works. The problem is that it's easy to make two separate models, one for inflation and one for dark energy. We have two new phenomena that require the introduction of at least two new "free parameters" for the theory to work. You can tie them together or not, but the models must not be distinguished.
Models of excessive evolution of dark energy (i.e. w≠-1 can be excluded from data)
All we've been able to do so far is exclude certain types of models in which the inflation rate is inconsistent with the observed values in the early or late stages of these models. But the observations are also consistent with inflation itself, with dark energy coming from completely different sources. I hate to give a full explanation of what we know, to have one phenomenon (inflation) occur in the energy range of about 10^15 GeV and another phenomenon (dark energy) in the energy range of about 1 millielectron volts, and then have to say "we don't know if they're related", but that's the reality.
Unfortunately, even if we had many missions to propose advanced astronomical telescopes such as James Webb, WFIRST, LISA, and ILC, we wouldn't expect answers from the data anytime soon. Our best hope is a theoretical breakthrough, and currently as scientists who specialize in the problem, we don't know how they got there.