How certain are we of the Universe’s ‘Big Freeze’ fate? (Synopsis)

"Even if I stumble on to the absolute truth of any aspect of the universe, I will not realise my luck and instead will spend my life trying to find flaws in this understanding - such is the role of a scientist." -Brian Schmidt

The discovery of the accelerated expansion of the Universe -- and of dark energy behind it -- in 1998 was one of the biggest physics revolutions of our lifetime. By measuring these distant galaxies and how their distances and redshifts scale in the Universe, we were able to determine that despite everything we knew about matter and radiation, there was an additional force at play, and it caused distant galaxies to accelerate away from us.

How matter (top), radiation (middle), and a cosmological constant (bottom) all evolve with time in an expanding Universe. Image credit: E. Siegel, from his book, Beyond the Galaxy. How matter (top), radiation (middle), and a cosmological constant (bottom) all evolve with time in an expanding Universe. Image credit: E. Siegel, from his book, Beyond the Galaxy.

The data is now good enough to determine that dark energy is extremely close, if not identical, to the predictions of a cosmological constant. But there are still other theoretical possibilities that are admissible, even if they aren’t favored. Perhaps in the coming decades, we’ll find out that the ‘Big Freeze’ isn’t necessarily where we’re headed in the distant future?

The different ways dark energy could evolve into the future. Remaining constant or increasing in strength (into a Big Rip) could potentially rejuvenate the Universe. Image credit: NASA/CXC/M.Weiss. The different ways dark energy could evolve into the future. Remaining constant or increasing in strength (into a Big Rip) could potentially rejuvenate the Universe. Image credit: NASA/CXC/M.Weiss.

We've got a lot of confidence in how far we've come, but a Big Crunch, a Big Rip, and a Cyclical scenario are all still in play!

More like this

"Quintessence is a dynamic, time-evolving, and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion [...] Whereas the cosmological constant is a very specific form of energy." -Robert Caldwell, inventor of the Big Rip scenario Our Universe began…
"If you're puzzled by what dark energy is, you're in good company." -Saul Perlmutter The accelerated expansion of our Universe was one of the biggest surprise discoveries of all-time, and something that still lacks a good physical explanation. While many models of dark energy exist, it remains a…
Some say the world will end in fire, Some say in ice. From what I've tasted of desire I hold with those who favor fire. But if it had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And would suffice. -Robert Frost Sure, if all we're talking about is the…
“Because dark energy makes up about 70 percent of the content of the universe, it dominates over the matter content. That means dark energy will govern expansion and, ultimately, determine the fate of the universe.” -Eric Linder It's been a while since we've spoken about dark energy, and we were…

You wrote "Neutrinos first act like radiation (a-4) and then like matter (a-3) once the Universe cools past a certain point." I would love to see further discussion of this process and how it happens.

By Glen Martin (not verified) on 18 Sep 2016 #permalink

@Glen Martin #1: The transition occurs only for massive neutrinos (the kind we have in our universe). For E >> m, the neutrinos are relativisitic (p ~ E), and you treat their energy density in the same way as photons: as the universe expands, their wavelength and number density both reduce, which gives the a^-4 dependence. If m_nu == 0, that remains true forever.

But in our universe, m_nu > 0, so at some point you reach E_nu ~ m_nu, or equivalently, E_nu(kin) < m_nu, p ~ mv. At that point, the neutrinos have so little kinetic energy that they start to behave cosmologically the way dust and atoms behave: they just bounce around locally like a gas, and as the universe expands around them their number density drops. This gives an a^-3 dependence.

By Michael Kelsey (not verified) on 18 Sep 2016 #permalink

@Michael Kelsey: Thanks for the clarification. Greatly appreciated.

By Glen Martin (not verified) on 19 Sep 2016 #permalink