Comments of the Week #65: from the highest temperature to the Earth's tilt

“We are a singularity that makes music out of noise because we must hurry. We make a harvest of loneliness and desiring in the blank wasteland of the cosmos.” -Jack Gilbert

Although you had lots to say, it came almost exclusively on two of our posts, so let's jump right in and see if we can expand what we've been talking about on your Comments of the Week!

Image credit: Ethan Shipulski, via http://mindblowingphysics.pbworks.com/w/page/52081285/Graviton%202012.

 

From Denier on temperature limits and singularities: "If at “some high temperature, you will restore the potential that caused our Universe to inflate, cosmically”, can’t that be used as proof that there are no physical singularities?"

This is an interesting argument: that if the creation of high enough energy densities is catastrophic and results in the restoration of the inflaton potential, doesn't that mean that the theoretical singularities formed at the centers of Schwarzschild black holes are a limiting factor for the Universe?

Image credit: SXS team; Bohn et al 2015.

 

It's an interesting thought, and I see where it comes from:

• singularities at the centers of black holes are supposed to be regions of infinite density,
• with all the mass of a black hole concentrated into an infinitesimal volume of space,
• and therefore they should have infinite energy densities,
• yet you just said, Ethan, if the energy density exceeds a certain finite amount, we'd restore the inflationary state and destroy the Universe.

Image credit: Moonrunner Design, via http://news.nationalgeographic.com/news/2014/03/140318-multiverse-infla….

 

It's a reasonable line of thought, but there are two things at play that prevent this from happening. One is cosmic censorship, or that all singularities are (mathematically) hidden from observers outside of an event horizon, and that pretty definitively prevents any information -- including inflating spacetime -- from getting out. And the second is the fact that singularities, or single particles in general, don't have a temperature in the sense that they can't just go and "create" stuff out of their existence. Temperature, as we define it, requires a system of particles, something lacking in a singularity. So I won't say that your notion is impossible, but I will say that this argument about an upper limit to the Universe's temperature does not constitute proof that singularities cannot exist.

Image credit: University of New Hampshire.

 

From Michael Hutson on the OMG particle: "The “Oh My God” ultra-high energy cosmic ray was estimated at 3 x 10^11 GeV. Ethan says that the Planck limit is 10^19 GeV, which corresponds to about 10^32 K, and the possible limit of triggering an inflation event lower than that at ~10^28 K. Would two OMG particles colliding head-on exceed that?"

I heard a story when I was in high school, researching the Reagan presidency and the Strategic Defense Initiative (or "Star Wars") programme. One of the researchers discussing laser defense -- the idea of shooting down an incoming ICBM with lasers -- talked about how current laser technology was three orders of magnitude too small to stop an ICBM. To which Reagan allegedly retorted something akin to, "well let's just send three of those suckers up there and let the Ruskis have it."

Image credit: © 2000-2015 GlobalSecurity.org, http://www.globalsecurity.org/space/systems/sdi.htm.

 

Only that would never work, because three order of magnitude is a factor of 1,000, meaning you'd need a laser 1,000 times as powerful (or 1,000 lasers at once) to reach that threshold. The OMG particle is down from the Planck scale by a factor of around 30 million, and down from the inflationary scale by at least a factor of ~3,000. Even if you got two OMG-type particles colliding head-on, it would "only" create about 6 x 10^11 GeV of usable energy, which is still well below the threshold necessary. It's interesting how cosmic rays do put a limit on what could destroy the Universe, but we're way below inflationary energies here.

Image credit: wiseGEEK, © 2003–2014 Conjecture Corporation, via http://www.wisegeek.com/what-is-cosmology.htm#; original fromShutterstock / DesignUA.

 

From John on the idea of a Big Bang and something before it: "From reading here and elsewhere, I had taken that “the Big Bang” = the universe at its very beginning. I thought that the expression “before the Big Bang” was meaningless, because of space-time starting with the Big Bang. And I’ve seen mentions of inflation as something that happened to the universe after the Big Bang."

This is one of the most common misconceptions out there, and it's because there are two ways of defining the Big Bang which used to be synonymous, but are no longer:

1. The hot, dense, expanding state that our observable Universe emerged from, that expanded, slowed, cooled, and gave rise to elements, atoms, stars, molecules, planets, and eventually us.
2. The initial singularity that represents the birth of space and time.

Image credit: ESA and the Planck collaboration, modified by me.

 

I wrote a huge piece on this relatively recently that delineates these two uses. While the first one is still valid, the second one may not be, and is certainly not a given and should definitely not be used interchangeably with the idea of the Big Bang that corresponds to the first explanation. Go read the whole story on The Two Big Bangs if you haven't before. (Or read it again, because it's good.)

Image credit: Masayoshi Matsumoto, via his tumblr at http://isopresso.tumblr.com/.

 

From PJ on realistic, balloon-only balloon animals: "Don’t like to mention it, but, baldie has been constricted in the wings with brownish twine, or nylon."

Look again: what you think of as "brownish twine" or "nylon" actually turns out to be... a balloon! This is a really clever way to get the desired effect while still adhering to the rules in both letter and spirit. Also... bald eagles don't tend to have the body of a lion and giant tails. I'm pretty sure that's a mythic creature known as a griffin.

Image credit: Bock et al. (2006, astro-ph/0604101); modifications by me.

 

From John Seal on inflation: "I’ve not read why the inflationary period began, why it lasted as long as it did, or why the inflationary period ended."

These are three good questions that we're not 100% sure of the answers to, so I'll give you the best answers we know of today:

1. We have no idea, but there's a quantum state known as the Bunch-Davies vacuum that could have given rise to an inflationary period from a previously non-inflationary state.
2. Inflation -- based on the models (like new/chaotic inflation) that work -- should have lasted an arbitrarily long amount of time in most regions where it occurs. If you want inflation to last long enough to create the Universe as we observe it, it pretty generically lasts a very, very long amount of time. This feature is part of the eternal inflation scenario. Maybe if you (or someone) asks nice, I'll do next week's Ask Ethan on "why is inflation eternal?"
3. This is the one bit we think we know: when the inflaton field reaches a certain critical value (for its value, slope and concavity), a process of reheating occurs, where the energy inherent to space itself gets converted into matter, antimatter and radiation.

All of these are currently active areas of research.

Image credit: Ned Wright's cosmology tutorial.

 

From Morgan on dark energy and flatness (excerpted): "[I]nflation made the universe flat, but that it inevitably produced the balance of components (normal matter, dark matter, dark energy) to keep it flat. Yet… isn’t dark energy going to come to dominate, and cause expansion to accelerate? Doesn’t that mean that the future shape of the universe is open, not flat/critical?"

There was a notion that held sway for a long time: that the matter density of the Universe determined both its fate and its curvature. If the density was below a certain amount, the expansion would continue forever and the curvature would be open; if it was above that critical amount, it would reverse and recollapse and the curvature would be closed; if it was at that critical value, the expansion would asymptote to zero and the curvature would be flat. But all of that was before we really considered the possibility of significant dark energy except as a toy model.

Image credit: NASA / NSF / DOE / AURA. (I think.)

 

Instead of having to live in an open Universe if the matter density was less than 100% of the critical value, we could live in a flat Universe if there was another form of energy density -- like dark energy (causing an acceleration) -- that made up the difference between the observed-to-be-low matter density and the critical value. With the discovery of the accelerated expansion in 1998, all the pieces came together beautifully. The big revelation is that in a dark-energy-rich Universe, curvature and fate are no longer tied together in the same way.

Image credit: Cosmic Inflation by Don Dixon.

 

And finally, from Sesh Nadathur on inflation's problems: "[P]oints 1 and 2 (otherwise known as “the flatness problem” and “the horizon problem”) are really more problems that inflation was designed to solve rather than predictions. They are also both really problems of the unlikeliness of initial conditions. E.g., the Universe could have just started off *extremely* close to flat due to random chance, without requiring a physical mechanism to explain it. However the odds against such a thing happening by chance are astronomically large: which is why an explanation involving a physical mechanism, such as a period of exponential expansion due to inflation, is appealing. The problem is how does this period of exponential expansion get started? If the fine-tuning required for inflation to start is just as extreme as the fine-tuning required for a “just-so” universe, then inflation has not really helped solve these problems, just pushed them further back in time."

Like a few other commenters here, Sesh is a bona fide theoretical physicist, although I disagree with a few things in his comments. I think it's problematic for inflation to deliver a low-density, almost-flat but open Universe, which would require fine-tuning either to get a small number of e-foldings (allowing the Universe to still be open) or a horrendously contrived model to allow for something stretched with a notable, consistent departure from flatness. This is why I think points #1 and #2 are so much more than merely a restatement of the horizon and flatness problems. One of them, in particular, states that the Universe should be indistinguishable from perfectly flat, something that was a problem at the time, with Omega_matter at ~0.2-0.3 or so and no knowledge of dark energy.

Image credit: NASA / WMAP science team.

 

For the last point, I think the question of "how did inflation get started" is a big red herring. Yes, we'd like to know, and we'd like to be able to work it out. But there are no potential observables possibly connected to this. By the very nature of inflation, it wipes out any information in the Universe prior to the final 10^-32 seconds (or so) of inflation, leaving no signatures in our observable Universe. At this point, inflation could have been eternal to the past, it could have come about through a cyclical state, or it could have begun from a non-inflating true singularity. (I wrote about this not too long ago.) But not only don't we know, we don't know how we'd find out. We'd like our theories to give better answers, but just because we don't know how to make this pre-observable-Universe part of inflation work doesn't pose a problem for the signatures and evidence that we do have of our inflationary past. It may be a portent of new physics or an omen of scientific advance -- these inconsistencies that Sesh alludes to -- but I don't think it's the dealbreaker problem that a few physicists describe. That's all for this week, everyone, and I'll see you back next week for more wonders and joys of the Universe!