There is something eerily similar in the origin of our universe, called cosmic inflation, and in the accelerated expansion of dark energy, which will ultimately decide its fate. This raises suspicions that these phenomena may be interrelated. This week, Andrew Gillett was asking his questions: if the theory of eternal inflation is true, then could dark energy be a harbinger of a return to this primordial state?
It's not just possible. This does not even require the theory to be correct. Let's start our conversation with the stage preceding the birth of the Universe in the form in which it is known to us - with cosmic inflation.
When the Universe we know, full of matter and radiation was born, it had several rather strange properties: spatially, it was flat, it had the same temperature everywhere, it had no remnants of ultrahigh energy, and it had very strange patterns in the form of regions with excess and reduced density. It is possible that the Universe was born already with these conditions. The theory of cosmic inflation is as follows: if the universe began with a period of exponential expansion, in which there was a huge amount of energy inherent in space, and then this period ended, then a hot Big Bang would have occurred already in the presence of all these conditions. It took a while to understand the implications and it took even longerto confirm the theory with data on fluctuations of the cosmic microwave background. But now cosmic inflation is considered the first and foremost of all that in the history of the Universe, which we can confirm with evidence.
Perpetual inflation is a consequence of inflation based on properties that we rarely think about. Usually, when there is a transition in nature, say, a pot of boiling water, the water in which passes from a liquid to a gaseous state, this transition process begins at different points. These dots expand and merge, creating large bubbles by the time they reach the surface. When we talk about water, we say that it boils. During the boiling process, small bubbles rise up and merge into larger bubbles by the time they emerge. But there is a problem with inflation. The areas where inflation does not end at a certain point in time continue to expand exponentially, and this does not allow those areas where it ended to “boil”. Therefore, the Universe we observe must be completely in one bubble,where inflation has ended, and not in numerous bubbles that boil together.
But at the far end of this spectrum, we see the fact that the expansion of our universe is apparently accelerating. The best explanation for this, based on our most accurate measurements, is that there is a small component of energy inherent in space, which we call dark energy. This energy component is omnipresent and evenly present at all points in space. And it is extremely small. If we convert it to mass according to Einstein's formula E = mc2, it will be equal to only one proton per cubic meter of the Universe. But the space is not only huge, it is also expanding! As time goes on, this dark energy becomes more and more important. Over time, after about eight billion years, it accelerates the expansion of the universe, and then becomes the dominant component of energy in the universe.
These two periods of inflation and accelerated expansion at a later stage may seem very different. The difference on these energy scales is simply colossal, it is 10 to the 120th power! But both periods represent energy inherent in space, both cause the matter in the universe to expand exponentially. And in the presence of time (fractions of seconds for inflation and a trillion years for dark matter), they will take everything that is not connected into a single structure in the Universe and spread it apart. There are many such models, and at their core, they all combine inflation and dark energy.
So, what are the chances that the universe will begin to recycle its formation? They are big
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1. If dark energy is indeed a cosmological constant, it could be residual energy from the inflationary period when it all started. If so, there is reason to say that in the presence of time it can weaken even more, moving into a new, much lower energy state. Perhaps such a transition will give an impetus to the emergence of a large number of particles of extremely low mass, such as neutrinos, axions, or to something even more exotic. These particles, in turn, will combine and create their own analogs of stars, planets, and maybe humanity on a sufficiently long time scale. If we cannot see this process, this does not mean at all that it is impossible. Maybe this is the fate that awaits our Universe in the very, very distant future, even if it will take googols of years.
2. Dark energy may not be a cosmological constant, but over time it can increase. If this is so, then it will grow and grow, which can lead to the scenario of a "big rip", when over time all connected structures in the Universe will rupture. But in this scenario, developed by Eric Gawiser, there is a possibility that at the last moment, just before space crumbles into oblivion, the inherent energy of space, indistinguishable from inflationary scenarios, will make the transition to the Big Bang! Such a scenario of a "revived Universe" not only can come true in our distant future. In it, our Universe may be much older than it seems. It is possible that it is infinitely old.
Well, now the evidence we have indicates that dark energy is indeed a cosmological constant. This means that scenario number 2 is excluded. If there is no lower energy state for the transition, then option No. 1 can also be excluded. But now we don't have enough data to reject even one of them. If I had a chance to place bets, I would say that a variant with a lower energy state is more likely. But the idea that dark energy is really constant and exists forever is better supported by the available data. Until we know for sure, we'd better not rule out any possible options. The Euclid spacecraft, NASA's WFIRST wide-angle infrared survey telescope, and finallyThe LSST Large Synoptic Research Telescope will help us measure dark energy even more accurately, and this will provide new evidence to support either the first or the second theory. And new discoveries in theoretical high-energy physics can tell us more about the first concept. Either way, Andrew, the answer to your question is this: dark energy may herald a return to the hot Big Bang from an inflationary state, but it does not depend on the eternal nature of inflation.but it does not depend on the eternal nature of inflation.but it does not depend on the eternal nature of inflation.
Ethan Siegel