Undergrad Paradox: How can we look back at the Big Bang?

[GreenWombat]

The Webb telescope images show galaxies that are so far away that it takes ages for their light to reach us, so long that we can see galaxies as they were soon after the big bang. How amazing - but hold on. If we came from the big bang, and nothing travels faster than light, how come we are here looking back at the light from our origin, the big bang?

 

[Orodruin]

The Webb telescope images show galaxies that are so far away that it takes ages for their light to reach us, so long that we can see galaxies as they were soon after the big bang. How amazing - but hold on. If we came from the big bang, and nothing travels faster than light, how come we are here looking back at the light from our origin, the big bang?

The Big Bang was not an explosion in space that sent things flying out from it. It happened everywhere. What you are seeing now as light from the early Universe is just the light released at that epoch at just the right distance to reach us now. Light from that epoch but closer already passed us and light from that epoch but further away will possibly pass us in the future (depending on the future expansion history and exactly how far away it originated).

 

[PeterDonis]

Moderator's note: Thread level changed to "I".

 

[Vanadium 50]

It's always a "paradox". Never "something I don't understand"

 

[GreenWombat]

Thanks. So the big bang "happened everywhere". Is this inflation or expansion of the universe? Can/did this inflation separate things faster than the speed of light? If so, we could wait for this ancient light to come to us - from all directions. Did this inflation only happen in the past for a short period? The universe is still expanding but is this still happening at such a great rate? Is there a good webpage describing this?

(Thanks also for lowering my "thread level". The question felt "advanced" to me.)

 

[berkeman]

(Thanks also for lowering my "thread level". The question felt "advanced" to me.)

The PF thread prefix indicates the level that you want responses to be. So when you set it at "A" = Advanced, that means you wanted responses at the graduate school / PhD level of discussion.

 

[Ibix]

Is this inflation or expansion of the universe?

The big bang happening everywhere is a feature of all modern cosmological models. They all expand, and if you run them backwards they all have energy density rising without bound. Inflation is a modification to the model to account for some specific features of the universe we inhabit.

Can/did this inflation separate things faster than the speed of light?

The recession speed of some distant objects is faster than light, yes.

If so, we could wait for this ancient light to come to us - from all directions.

Light from what? The Big Bang? We already see the do called "last scattering surface" which is the time when the universe became transparent - it's the cosmic microwave background (CMB).

The universe is still expanding but is this still happening at such a great rate?

Expansion continues, and the rate is characterised by the (slightly misleadingly named) Hubble constant. It was higher in the past.

 

[artis]

The Webb telescope images show galaxies that are so far away that it takes ages for their light to reach us, so long that we can see galaxies as they were soon after the big bang. How amazing - but hold on. If we came from the big bang, and nothing travels faster than light, how come we are here looking back at the light from our origin, the big bang?

It is because of the dynamic nature of spacetime. If you separate a distance between two light sources then the further those sources get the longer it takes for light to travel from one to the other.
Even to the point where if those objects are far enough from one another light never reaches the other object because due to accelerated expansion the distance between them increases faster than light speed.

The c speed limit of light is only valid for matter within space( matter/atoms/particles can't travel faster than c ) but not for space itself, space itself can exceed that limit in terms of expansion which we indirectly observe as the increase in distance between distant objects (directly observing the redshift of light)

 

[kimbyd]

(Thanks also for lowering my "thread level". The question felt "advanced" to me.)

Think of "advanced" meaning, "I really want to see a bunch of detailed equations," which I don't think is what you were going for. Usually it's used by graduate students or former graduate students.

 

[kimbyd]

Thanks. So the big bang "happened everywhere". Is this inflation or expansion of the universe? Can/did this inflation separate things faster than the speed of light? If so, we could wait for this ancient light to come to us - from all directions. Did this inflation only happen in the past for a short period? The universe is still expanding but is this still happening at such a great rate? Is there a good webpage describing this?

Currently most of the galaxies we can observe are now and always have been receding at faster than the speed of light. Fundamentally this is because in General Relativity, the curvature of space-time mucks up any notion of far-away speed: relative speed can only be defined precisely for objects passing one another. This means that the speed of light limitation in General Relativity is a local limitation: nothing can outrun a light ray.

Get far away, though, and the curvature of space-time intervenes. Because we can no longer even have a unique definition of a far-away speed, the speed of light limitation cannot apply.

Instead there are multiple possible definitions of far-away speed. The most commonly-used one is recession velocity, which is the rate of change in the proper distance between two objects, using a time slicing where the CMB is the same temperature for each. This is all complicated because curved space-time makes for a lot of ambiguity. And using this definition, which is probably the most intuitive one, recession velocities for all galaxies more than a couple billion light years away are now and always have been greater than the speed of light.

 

[Maarten Havinga]

ATM it is unfeasible to test if inflation could have happened, let alone experimentally verify that it has happened. People bring it up to explain the CMB, and the origin of the universe without the 'problem' of an apparent intelligent design (which is not to be discussed on this forum for too much being religion, but I'm now discussing inflation).

Because inflation is unfalsifiable with our current equipment, it's a matter of belief - the explanation offered by the Big Bang theory, which requires inflation, or whatever you find plausible instead of inflation - requiring a different first second of the universe and lots of good arguments in order to earn a place in the scientific community. It should at least include an (alternative) explanation of the CMB, and that's difficult.

For myself, I dislike assumptions such as inflation (why did it start? Why did it stop? Why is it not reproducible? Where is the inflaton particle?). Besides that, it's freaking HARD to say whatever happened 13.8 billion lightyears away, be it in time or in space. But if religion cannot be discussed, I cannot discuss alternative explanations here.

 

[PAllen]

Look, while this example I will provide has nothing to do with realistic cosmology, it is enough to show there “is no paradox there”, as it involves only special relativity. Suppose a partner, at some moment, starts moving away from me at .99999999…9 c. Suppose I can see a clock they carry. Then after a million years of my time, I might be seeing their clock showing only one second after they departed. This framework can be formalized into the so-called Milne cosmology which is all SR, no inflation or GR at all. It has almost all features of realistic cosmology qualitatively, but not quantitatively. I find it very useful for the purpose of showing that neither inflation, nor curvature, are necessary for the most common cosmological phenomena to occur.

Th Milne cosmology also demonstrates that recession velocity is a different quantity altogether than relative velocity. Recession velocity in the pure SR Milne cosmology is unbounded, while relative velocity is unambiguous and less than c. The GR analog of the latter statement is that if you parallel transport two 4 velocities to a common event to compare them, the result of the comparison is path dependent but always less than c,

 

[Orodruin]

.99999999… c

So ... c?

(I know it is being pedantic, but 0.999... = 1)

 

[PAllen]

So ... c?

(I know it is being pedantic, but 0.999... = 1)

Fixed.

 

[Drakkith]

Because inflation is unfalsifiable with our current equipment, it's a matter of belief

I think it's a bit strong to call it 'belief'. Inflation is, after all, a tentative explanation that's widely recognized as being very incomplete and lacking concrete evidence.

 

[Maarten Havinga]

I can also call it a matter of whether you like the idea or not, is that better?

As long as we cannot place an inflaton particle in String Theory, I won't accept the idea of inflation. Even dark matter is less hypothetical.

 

[Orodruin]

String theory, if anything, is hypothetical.

 

[Maarten Havinga]

Well it gives clear predictions and insight in very unlikely theory connections at least. And it's a falsifiable theory if you pick 1 configuration, approximate the calculations and check with reality.

WIMPs were one prediction from string theory, and the LZ experiment to some extent falsified them. String theory gives insightful leads, even if hypothetical.

 

[Orodruin]

if you pick 1 configuration

Enough said …

 

[Maarten Havinga]

Enough said …

The same goes for Quantum Field Theory, only there we know 1 configuration gives the Standard Model.

 

[Drakkith]

I can also call it a matter of whether you like the idea or not, is that better?

What does that matter? There are plenty of tentative explanations in science. One can support that such an explanation is better than others without subscribing to 'believing' in it.

As long as we cannot place an inflaton particle in String Theory, I won't accept the idea of inflation. Even dark matter is less hypothetical.

What does string theory have to do with this?

Well it gives clear predictions and insight in very unlikely theory connections at least. And it's a falsifiable theory if you pick 1 configuration, approximate the calculations and check with reality.

First and foremost, inflation is absolutely falsifiable. It's been around for over 40 years, since before even the first CfA redshift survey (the first attempt to map the large-scale structure of the universe) was published. This survey, along with a great many other observations since then, could have easily falsified inflation by finding that the universe is not as homogenous and isotropic at large scales as we thought. Any one of these increasingly precise observations could have spelled doom for inflation.

In fact, here's a 100-page paper explaining many details of inflation, including how it works and some of its experimental verifications: The Observational Status of Cosmic Inflation after Planck

The observational status of inflation after the Planck 2013 and 2015 results and the BICEP2/Keck Array and Planck joint analysis is discussed. These pedagogical lecture notes are intended to serve as a technical guide filling the gap between the theoretical articles on inflation and the experimental works on astrophysical and cosmological data. After a short discussion of the central tenets at the basis of inflation (negative self-gravitating pressure) and its experimental verifications, it reviews how the most recent Cosmic Microwave Background (CMB) anisotropy measurements constrain cosmic inflation. The fact that vanilla inflationary models are, so far, preferred by the observations is discussed and the reason why plateau-like potential versions of inflation are favored within this subclass of scenarios is explained. Finally, how well the future measurements, in particular of B-Mode CMB polarization or primordial gravity waves, will help to improve our knowledge about inflation is also investigated.

So inflation is unfalsifiable? No. Not at all.

 

[Maarten Havinga]

From a homogeneous initial condition, inflation is constrained by CMB anisotropy yes. But if the universe was less homogeneous, you only need to change the initial condition. In fact Penrose calculated the possible inflationary scenarios through and got the answer that an inhomogeneous universe is very well possible with inflation. This is an understatement

 

[Orodruin]

The same goes for Quantum Field Theory, only there we know 1 configuration gives the Standard Model.

You are completely missing the point. Calling out inflation as being devoid of evidence and then using being able to include it in a string theory as a model criterion is about as much of a pot calling a kettle black as I can think of. Can you give one good experimental reason to use this as a criterion?

String theory in its early incarnations was all about finding a single viable theory that would explain everything. It has fallen quite far since. Do not get me wrong, string theory may be true or not. I just think it is a bit impudent to claim to use it as a selection criteria.

Edit: Also note that saying that particular configurations are falsifiable is not the same as saying that string theory is falsifiable. What would it take for you to conclude that string theory is false?

 

[Maarten Havinga]

Do not get me wrong, string theory may be true or not.

Thanks :)

Can you give one good experimental reason to use this as a criterion?

Perhaps not experimental but: Look at how quantum physics gave an explanation and a recipe to follow that a chemical element must obey. If there's any theory that has the potential to explain "why the Standard Model?" and similarly give a framework for elementary particles (leaving space for the graviton and other particles we have not yet found) by giving criteria for a particle to exist, it is String Theory. So in deciding if an elementary particle or quantum field can exist, String theory is right now the best judge we can produce. Perhaps new ones will develop, related to the thread on three generations of fermions from Octonions. Then it could become a different matter for me.

Don't misunderstand me, I'm not saying anyone should use this criterion - I'm saying I use this criterion for my own reasons to investigate or not.

 

[Orodruin]

Don't misunderstand me, I'm not saying anyone should use this criterion - I'm saying I use this criterion for my own reasons to investigate or not.

That is very different from the statement that you started with:

As long as we cannot place an inflaton particle in String Theory, I won't accept the idea of inflation.

This seems to be saying that you simply will rule out, without further thought, any theory that cannot be embedded into a string theory.

 

[Maarten Havinga]

I meant to accept as: to believe it could have happened. And one can easily not believe something without any requirement for others to not believe.

 

[artis]

@Maarten Havinga I believe what @Orodruin meant to say is that inflation might have never happened but whether it happened or not has nothing to do with String theory.

In fact I would say that whether something is real or not has nothing to do with any theory, even with the ones we accept as generally true, simply because nature happens without our permission so best we can do is find good explanation of how it happens and then hope that with time and experimentation the hypothesized results don't change.I recall there was a saying that "all you need is just one wrong prediction/result for a theory to be wrong"
let's say spring theory somehow proved inflation, does that make string theory true? Or does it make it more true than the standard model?
Mathematical beauty on paper means absolutely nothing unless it matches observation 1:1

 

[malawi_glenn]

As long as we cannot place an inflaton particle in String Theory, I won't accept the idea of inflation

What experimental and observational evidence do we have in favor of string theory?

 

[Orodruin]

And one can easily not believe something without any requirement for others to not believe.

Sure, but then we are not much further along than religion and no longer discussing science.

 

[Vanadium 50]

What experimental and observational evidence do we have in favor of string theory?

These theories are supersymmetric, and we've already found half the particles!