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Comment by willis936 | Hacker News Reader

I'm so excited for JWST's data. Observations from the inflationary period have the potential to completely upend cosmological dogma. Findings like this are the precursor. I'm sure theorists are furiously working on alternative models. This is an exciting period to watch.

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Inflation happened during the first tiniest fractions of a second post big bang. No telescope is going to make direct observations of the inflationary period so I'm not sure what you mean by this.

Well, I guess measurements of the early universe (100-250 million years after the big bang) can test predictions made by models of the inflationary period.

Measurements of pretty much any time in the universe can test predictions made by models of the early universe. One of the main reasons we think there was inflation is from late time (near today) observations of matter density (see https://en.wikipedia.org/wiki/Flatness_problem).

I'm referring to the earliest era of transparent space, when conventional matter supposedly coalesced. If we look back to that period and still see red-shifted mature galaxies then something is very wrong with the current models. I'm under the impression that we don't have good observations of the period 375k-400M years after inflation and that infant galaxy observations have not been confirmed.

The earliest moment of transparent space is the CMB. I know JWST can't detect Pop3 stars, but is there even expectations that it could detect proto galaxies with those stars? I thought it was still much later, like the early Pop2 generation.

> No telescope is going to make direct observations of the inflationary period so I'm not sure what you mean by this.

Isn’t this an engineering problem and what we are attempting to do with gravitational wave detectors?

I know at one time it was believed gravitational waves were detected that provided direct evidence for inflationary theory but then the data was determined to be dust from the Milky Way. I thought this was still one of the major ongoing efforts in gravitational wave detection, was this ruled out?

But those aren't really "telescopes", are they? :)

Yes gravitational wave telescopes is commonly used along with gravitational wave detectors, gravitational wave instruments and gravitational wave observatories.

This isn't just an engineering problem. Photons couldn't move around for the first 380K years or so. Space was nearly uniform, hot, and dense. The CMB is literally the heat wave left over from the point where the heavy soup thinned out just enough to allow photons to fly away in all directions.

The gravitational waves from those events would have already warped space, and the ones just now reaching us would be from the edge of the observable universe, and so too weak for any instruments we could conceivably build in the next few decades. Not that we shouldn't try, mind you. There are new frontiers in quasimatter and time crystals that could yield far more accurate gravitational wave detectors.

Also fascinating would be to attempt to decipher the deformations left in the metric already. There are some theories that basically say gravity waves permanently "crumple" spacetime, and it might be possible to read signatures of such events if this is so.

> There are some theories that basically say gravity waves permanently "crumple" spacetime

Does that mean those theories predict you can achieve permanent gravitational effects (locally) without any matter or energy to cause it? Wouldn't that violate relativity?

No, you still need something to create the gravity waves. The idea is that once a gravity wave has passed through a region of spacetime it leaves a permanent deformation. In the case of a detector like LIGO this means rather than the mirrors wobbling relative to each other and then settling down to their previous configuration, in fact they are left in a (very slightly) permanently altered configuration. The difference is probably way too small to detect though, for now anyway.

I understand what you're writing (I think), but I don't think I understand how the implications would be consistent with relativity unfortunately. I thought gravity is (supposedly) caused by the deformation of space. If your space isn't flat, then you're going to experience acceleration (aka gravity) at that point, right? And if this deformation is permanent, then its source is already long gone - meaning that when you look down to see why you're falling, you see that there's no matter or energy causing you to fall. Which seems weird to me because I thought you need some kind of matter/energy to cause space to curve (and hence feel gravity/acceleration). Is that not the case?

These distortions are predicted by relativity. They’re a consequence of it, not a problem with it.

Objects influenced by a gravitational field don’t feel an acceleration. Astronauts in orbit round the earth don’t feel anything even though they are going round in circles. It’s only when something gets in the way that you feel an acceleration from the thing stopping your trajectory, like the surface of the earth.

The distortions were talking about are created by a mass though, the mass that created the original gravity wave.

> Objects influenced by a gravitational field don’t feel an acceleration.

We don't feel velocity but we do feel acceleration, whether it's due to gravity or anything else. I still don't follow the logic unfortunately.

You don't feel acceleration either, you feel the normal force of an object against you. If you're in freefall you feel nothing (or really, you do feel the absence of a normal force you're used to).

However, that's not what the comment you replied to was saying. Gravity doesn't apply a force at all! A force is when your worldline gets pushed around, but an object undergoing gravity is actually still going "straight", it's spacetime that's deformed.

Ohh, right! Interesting! I think I'm finally seeing what's going on. Thanks!

One problem is that a lot of simplifications get layered on in pop science explanations of relativity, or actually deep science of any kind. We often talk about the force of gravity, but the thing is it's actually not a force, or even a field, in the same way as other forces and fields in physics.

This is why Einstein is so revered in Physics. He didn't just explain a force, otherwise why would he be given pre-eminence over Maxwell who explained electromagnetism? Relativity is something else completely.

Here's a really good explanation of what's actually going on when gravity influences an object. Hold on to your chair.

https://www.youtube.com/watch?v=UKxQTvqcpSg&t=19s

> it's actually not a force, or even a field, in the same way as other forces and fields in physics

That's why (for anyone who's still reading this old thread) gravity is treated separately from the other three "fundamental" forces. We can explain pretty much everything in physics with the tools of Quantum Field Theory, even if there's still some gaps, but relativity is a whole other ballgame. Not only is it totally resistant to the mathematical techniques (quantum operations are linear, relativity is very much not, to name just one issue), but it's sort of unclear what QFT looks like without spacetime as a stage to act on. If the properties of spacetime emerge from some simpler, presumably more quantum, system, we have next to no idea how.

The deformations in spacetime are permanent (hypothetically). The mirrors in LIGO, however, are subject to all the other various things in the environment tugging on them. They return to a state of alignment because of their environment (local gravitational forces, etc.) This "noise" would prevent a detector like LIGO from seeing the permanent changes to spacetime.

https://www.quantamagazine.org/gravitational-waves-should-pe...

https://news.ycombinator.com/item?id=29485803

  > they are left in a (very slightly) permanently altered configuration

Might this explain the cosmic filaments? The concentrations of matter might be these very distortions.

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