Cosmic Inflation’s Five Great Predictions (Synopsis)

“Scientific ideas should be simple, explanatory, predictive. The inflationary multiverse as currently understood appears to have none of those properties.” -Paul Steinhardt, 2014

Cosmic inflation is alternately talked about by serious scientists as either the definitive beginning to our Universe, the thing that happened before and set up the Big Bang with absolute certainty, or a speculative fiction that can never be falsified, leading to nothing but untestable predictions and things that only mattered after-the-fact of their discovery.

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

But inflation has five unique predictions that it made intrinsic to all (reasonable) models back in the 1980s, before any of them were known:

  1. A Flat Universe,

  2. A Universe with fluctuations on scales larger than light could’ve traveled across.

  3. A Universe whose fluctuations were adiabatic, or of equal entropy everywhere.

  4. A Universe where the spectrum of fluctuations were just slightly less than having a scale invariant (n_s < 1) nature.

  5. And finally, a Universe with a particular spectrum of gravitational wave fluctuations.

Image credit: National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by me. Image credit: National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by me.

How did it do? Come find out on the five predictions of cosmic inflation!

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I've not read why the inflationary period began, why it lasted as long as it did, or why the inflationary period ended.

By John Seal (not verified) on 17 Jun 2015 #permalink

I'm curious about point one, the flatness of the universe. I understand (I think) the idea that inflation stretched the universe flat. However, this post seems to be saying not just that inflation 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? It reads like inflation is being credited for giving us an apparently finely-tuned universe, balanced on a knife-edge, as an inevitable side effect of expansion... but in fact the balance is a temporary thing, strictly contingent, and I don't see how it saves us from anthropic arguments, which I know Ethan doesn't like.

Am I misunderstanding something? Perhaps I'm mixing up different senses of open/flat/closed?

Morgan, I believe you're missing "adiabatic" there.

Adiabatic means you don't lose energy, usually because it happens too quickly for other processes to remove that energy in or out of the system.

"Fast" is the case in inflation. It did happen very quickly.

It's slowing now. Very slow. Adiabatic isn't the case any more.

Therefore the case of flatness in the adiabatic period is no longer a consequence.

But... "adibatic" isn't used in the section I'm talking about - point 1. Point 1 talks about:

The critical density of the Universe, or what the matter density would need to be to keep the Universe perfectly balanced between the recollapsing eventually case and the expanding forever case.

as the state that inflation predicted, but that observations at the time of the prediction didn't match. So is it just a historical matter, then, that inflation predicted the universe had been flat up to now, when our observations used to indicate it probably hadn't been? And the component that happens to balance that out to match the prediction is one that will make it unflat in the future?

"But… “adibatic” isn’t used in the section I’m talking about "

Yeah? So? All that means is you're no longer missing it, you're ignoring it.

This isn't an improvement. Quite the opposite. By a shitload of negative intelligence.

What you refuse to acknowledge is adiabatic. When you start acknowledging it, then you can start answering your question yourself. You obviously don't want anyone else to do it.

Please consider the possibility that your explanation wasn't as clear as you thought it was, and your reaction there completely uncalled for. Hostility aside, it's very strange to say I don't want answers when my reply sought clarification as to whether I was understanding your answer correctly.

I did consider that. And considered it invalid.

Now, about you: what consideration did you give before posting? Any at all?

How could you claim that you think my explanation wasn't clear when you clearly said you didn't consider "adiabatic" of any point to you at all?

Or have you considered what I said in post 2 and gone "Oh, I see!"? In which case where is the withdrawal of your petty complaint in 3?

I'll attempt to clarify where I am/was coming from, since I think you've gotten the wrong impression.

You said "I believe you’re missing “adiabatic” there." I read this, perhaps incorrectly, as suggesting I'd overlooked a use of "adiabatic" in the section I was confused by, that would make it clearer if I found it. I looked, I didn't find it, I reread the section, I tried to understand how what you'd said in #2 bore on my questions about it, and I asked whether my best read of what I thought you were saying was in fact correct. Since you didn't answer that question in your replies since, I assume it wasn't.

So, okay. If you think your explanation there was obvious and sufficient, such that if it isn't so to me then there's no point trying to explain it further, you're completely free to simply leave it be. Just be less hostile and insulting about it, please.

Morgan - you're dealing with a troll. I wouldn't waste more time with it.

In fact your question is perfectly reasonable. The answer is that in fact the Universe doesn't remain close to flat - any small deviation from flatness far back in time is massively amplified by "normal" cosmic evolution (irrespective of the precise balance of components you mention). So from the fact that the Universe looks very close to flat today, we can deduce that it must have started off very, very, VERY close to flat far in the past.

How did it get that way? A period of accelerated expansion (such as arises from inflation) drives the Universe towards flatness (whereas "normal" evolution drives it away from flatness). Therefore a sufficiently long period of inflation in the past could explain why the Universe started off sufficiently close to flat that the curvature is still undetectably small today.

By Sesh Nadathur (not verified) on 18 Jun 2015 #permalink

So you didn't read the topic? This was at the start:

But inflation has five unique predictions that it made intrinsic to all (reasonable) models back in the 1980s, before any of them were known:

A Flat Universe,

A Universe with fluctuations on scales larger than light could’ve traveled across.

A Universe whose fluctuations were adiabatic, or of equal entropy everywhere.

So how do you manage to get a question about the subject when you skipped the bit before the section you are asking for?

While Ethan's article is perfectly true - these are all fundamental predictions of inflation - a little bit of context is also necessary.

For instance, points 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. And it is the case that all existing models of how inflation might actually be realized in a particle physics scenario do require this fine-tuning.

Predictions 3 and 4 are more genuine wins for inflation. The problem here is that these predictions are a little too generic or vanilla, in that one could imagine alternative scenarios which could *also* produce nearly-scale invariant adiabatic perturbations (and in fact many people do). Therefore while the success of these two predictions is important, it doesn't quite constitute a smoking gun.

It's only prediction 5 that could be a smoking gun - a gravitational wave background with that specific spectrum could only be produced by inflation. Unfortunately there is enough leeway within the inflationary paradigm for different inflation models to play with the magnitude of the signal, so inflation doesn't guarantee a *detectable* gravitational wave background. This fact is sad, and upsets some people, but such is life.

I would summarise the situation as: the data support the hypothesis of a period of exponential expansion in the early Universe; we don't yet have a completely convincing theoretical framework for explaining the cause of this expansion; alternative explanations of the data exist but are less compelling.

By Sesh Nadathur (not verified) on 18 Jun 2015 #permalink

By making a careless oversight when I did read it. I apologize for that.

I think my understanding of your explanation is correct, then. Glad to have that cleared up.

But again: please, look at how you've responded to me here and ask yourself, seriously, if the assumption of stupidity and bad faith you seem to be making is a reasonable or warranted one. Given what the actual point of confusion on my end turned out to be, would it perhaps have been simpler to say "the part I'm talking about occurs before the part you were looking at"?

Actually, please ignore my last paragraph there. My question's been addressed, and there's no point dragging this out into a discussion of your commenting style.

What's actually quite amusing about this is that Wow's answer has nothing whatever to do with the question. In fact it is completely nonsensical, whereas the question about flatness is perfectly coherent and reasonable - and is unrelated to adiabaticity.

By Seshadri Nadathur (not verified) on 18 Jun 2015 #permalink

What's amusing is that you're wrong!

I LoL'd.

Do you know what the question was?

It's obvious that Wow & Morgan are in love and should just get a motel room and stop this lovers spat and move on.

By mitru costea (not verified) on 18 Jun 2015 #permalink

"In fact your question is perfectly reasonable"

Aaand where did I say it wasn't?

Nowhere.

I answered it, after all.

I suspect you just want to claim someone is a troll so that you can have a cheap thrill.

"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."

Hmmm. You never say how inflation does that, though.

Nor do you actually answer Morgan's question. You seem to be pandering to them, though when yuu say this:

"And it is the case that all existing models of how inflation might actually be realized in a particle physics scenario do require this fine-tuning."

Nope. Random chance will have some small region flatter than the average of the whole. And inflation will make what we see today a small part of the whole. Therefore seeing a very flat bit isn't anything special at all: there could be an indefinite, but huge, number of places where this happens.

There is no need for it to be fine tuned.

Just variable.

It's obvious mitru is making shit up to appear "smart".

t'aint workin' kid. Opposite.

@ WOW #15
Excellent reference .......

I think that for some people, PJ, the explanation doesn't stick because they're not making any connections to the subject to which to attach the memory of the explanation they demand.

Even looking independently for a few minutes beforehand can multiply the connections available for an answer.

That link gives some structure on what to do BEFORE asking a question. And far better than I could or would.

It's not that people are just too dumb to understand (at least to the level you can manage on a blogpost),but that they'd prefer to be dumb if it is going to take *effort* to rectify that situation.

A bit like Calvin and Hobbes when Calvin's dad explains that wind is from sneezing trees. When Calvin asks if that's true, his dad says "Well, no, but the truth is harder to understand". The final panel has Calvin say "Boy, the trees sure are sneezing hard today".

Because being dumb about something is, frequently, a choice made out of laziness.

Or possibly a fear of being wrong.

Being wrong isn't a sin.

Wanting to remain one is the closest humans get to a sin of thought.

Yes, pretty well sums it up .....

Sesh: Thanks, but that actually adds to my confusion. You talk about flatness as an observed fact that inflation explains, but the post talks about it as a prediction made by inflation that didn't match the data at the time.

Morgan saw you hadn't answered them. Making post #16 ironic indeed.

Morgan, he's incorrect in his assertion about inflation.

extraordinary claims about vast multiverses and a lot of other things that will never ever be possible to throw out if allowed in,
..require extradinary super duper multiverses full up to the top of evidence. Those 4 or 5 aren't sufficient for the claims.

By Chris Mannering (not verified) on 19 Jun 2015 #permalink

Yes, they do require extraordinary evidence.

Which is what they're trying to do. Until then, it won't be scientifically settled that we do have multiverses.

Where, precisely, is your issue here?

"Those 4 or 5 aren’t sufficient for the claims."

Those 4 or 5 aren't trying to prove mulitverses. They have nothing to do with multiverses, only with inflation.

And they don't prove it, they are what any theory of what happened at the early stages of the universe have to explain, which inflation does and none of the other contenders managed to do.

I believe your problem here is you don't understand the science, or any science discussion, but wish to argue that it's all bunkum because it makes you feel better about yourself to do so.

Off topic question here: what prevents photons from forming during the universe's first microsecond?

Nothing prevented it. They DID so form.

A Universe with fluctuations on scales larger than light could’ve traveled across.

A Universe whose fluctuations were adiabatic, or of equal entropy everywhere

if light hasn't yet travelled the distance of one fluctuation then you can't discuss fluctuations (plural) and whether they're adiabatic or not, especially if attempting to speak for "everywhere".

or can we now see all these fluctuations that were supposed to be beyond the horizon?

By questioner (not verified) on 22 Jun 2015 #permalink

You seem to have the wrong handle. "Proclaimer" would appear more accurate.

The light from each place can reach us faster than they cross that angular distance because it's a shorter distance for them to cross to us than to go to each other.

Adiabatic means that you've not lost any energy or equivalently gained extra entropy. This is orthogonal to any complaint you've made about the two areas.

Moreover, your "alternative" doesn't ensure that this would solve any issue about the claims Ethan made.

"the light from each place can reach us faster than they cross that angular distance because it’s a shorter distance for them to cross to us than to go to each other."

if we model this as a circle (the fluctuation) with an observer at the center of the circle, the light (or signal) may traverse the distance from the perimeter to the center in half the time it would take to travel the full distance of the diameter and/or less time than from distance inside the confines of the neighboring fluctuation.

if the argument is that you need a travel time for the signal that is less than the diameter of the local fluctuation then you can only speak for what you have detected within the confines of the local fluctuation...or extending at most to a short distance within the few surrounding fluctuations.

proclaimer? maybe? nice to talk with you Wow.

not sure what you mean by "my alternative" or what Ethan claims...just looking at the situation of fluctuations and distance.

By questioner (not verified) on 22 Jun 2015 #permalink

"if we model this as a circle"

This has no bearing on this claim:

"if the argument is that you need a travel time for the signal that is less than the diameter of the local fluctuation then you can only speak for what you have detected within the confines of the local fluctuation"

Nope. Not unless you're making "local fluctuation" being "everything since the big bang". Remember, distance radially on this circle you have modeled is travel to the past.

In which case, the response would be "So what?" Since all you're saying is that we can only say what has happened up to the current date. Which is tautological and nothing approaching a problem (of which you have stated nothing).

Yes, by definition of "up to now", we can only lay claim to what has happened up to now, and the future would be a projection or prediction.

The lack of evidence for the future having already happened is not anything to claim a problem over.

"not sure what you mean by “my alternative”"

Then I'm not sure what you meant by

"or can we now see all these fluctuations that were supposed to be beyond the horizon?"

“if we model this as a circle”

This has no bearing on this claim:

“if the argument is that you need a travel time for the signal that is less than the diameter of the local fluctuation then you can only speak for what you have detected within the confines of the local fluctuation”

Nope. Not unless you’re making “local fluctuation” being “everything since the big bang”. Remember, distance radially on this circle you have modeled is travel to the past.

In which case, the response would be “So what?” Since all you’re saying is that we can only say what has happened up to the current date. Which is tautological and nothing approaching a problem (of which you have stated nothing).

Yes, by definition of “up to now”, we can only lay claim to what has happened up to now, and the future would be a projection or prediction.

The lack of evidence for the future having already happened is not anything to claim a problem over.

the discussion is about form, size and detectable limits of fluctuations.

i am responding to the premise':

"2.A Universe with fluctuations on scales larger than light could’ve traveled across"

you added:

The light from each place can reach us faster than they cross that angular distance because it’s a shorter distance for them to cross to us than to go to each other.

angular distance of what? what are you talking about?

By questioner (not verified) on 22 Jun 2015 #permalink

lol

epic fail

goodbye

By questioner (not verified) on 22 Jun 2015 #permalink

You're talking about yourself, right?

Or was that just a flounce?

nope, wrong again

By questioner (not verified) on 22 Jun 2015 #permalink

there was a website called bad astronomy which a few years ago became "cosmoquest".

i hung out at bad astronomy for many years and if anyone had pulled a stunt like you did with that wiki link, they would have been made a laughing stock.

why don't you go there? plenty of "peers" that will be ready willing and able to bite your head off and spit down your neck the moment you make a mistake.

By questioner (not verified) on 22 Jun 2015 #permalink

tautology?

the future?

you're not worth talking to.

goodbye.

By questioner (not verified) on 22 Jun 2015 #permalink

"nope, wrong again"

oh, fair enough, you weren't confused, just lying.

Got it.

"you’re not worth talking to."

Well, a conversation does require you to attempt to actually engage in it. Which you preferred not to.

Not my problem, though, so no worries.

no, you're the liar.

i did converse.

there's no rule that says i have to continue a conversation when i find the person to be an idiot.

By questioner (not verified) on 23 Jun 2015 #permalink

your comments now are just attempts to divert attention away from the fact that you let your big fat ugly ego run rampant.

you're the only one who thinks you can keep putting lipstick on a pig and come up looking sweet.

everybody else can see what you're like regardless of whatever effort you make to blame others for your ugly mind.

By questioner (not verified) on 23 Jun 2015 #permalink

you've got your hat-trick now. thats a real losing streak.

By questioner (not verified) on 23 Jun 2015 #permalink

i'm leaving now.

but you can put another layer of lipstick on and see if anybody thinks your pretty.

By questioner (not verified) on 23 Jun 2015 #permalink

"no, you’re the liar.

i did converse."

No, a stream of words isn't conversation. There has to be meaning intended.

You've decided just to blabber. That isn't conversing.