Undergrad How Does Space Expansion Affect the Velocity of Galaxies?

[phinds]

That is not a rate, that is a speed. The distance between two objects 10 Mpc apart grows by 733 km every second.

 

[phinds]

That is not a rate, that is a speed. The distance between two objects 10 Mpc apart grows by 733 km every second.

And after approximately 30,000,000,000 years (twice the current age of the universe) they will be 20mpc apart and receding at 1466 km / sec

 

[Delta2]

The velocity due to expansion of space we call it recession velocity. How do we call the "real" velocity due to real motion?

 

[phinds]

The velocity due to expansion of space we call it recession velocity. How do we call the "real" velocity due to real motion?

Proper motion.

 

[Delta2]

Proper motion.

What is the cause of expansion of space and what is the cause of proper motion?

 

[Delta2]

here is my take;
Cause of expansion of space: Gravitational Field, Dark energy +?
Cause of proper motion: Gravitational field (from ordinary matter or dark matter) , kinetic energy released from explosion of stars?, Thermal motion of gas nebula? What else?

 

[PeterDonis]

here is my take

If you're going to ask the question, you should wait for an answer instead of indulging in speculation on your own.

The answer is that the distinction being made is not a distinction of "causes". The distinction between "expansion of space" and "proper motion" is coordinate-dependent and is not a matter of actual physics. For convenience, cosmologists pick out a particular class of worldlines called "comoving" worldlines, and say that the increasing spatial separation of those worldlines with time is "expansion of space" while motion relative to those worldlines is "proper motion". But that is a distinction of convenience and does not mean that those two things are two physically different things with physically different causes.

In terms of causes, there are two causes that are relevant for the motion of any object in the universe: the universe's spacetime geometry, and the 4-velocity of the object. This has nothing whatever to do with the distinction between "expansion of space" and "proper motion".

 

[Ibix]

Can we / should we extrapolate that the rate of expansion of space, of a universe that is / may be infinite, is potentially infinite?

Well, the point about the universe outside our observable region is that we cannot see or affect it and it cannot see or affect us. So there's a sense in which modelling it is pointless. However, when we assume that the universe is the same everywhere ("spatially isotropic and homogeneous") we get a result that looks very similar to what we see. Occam's razor suggests that we should assume that the rest of the universe that we can't see is the same, because to believe otherwise we'd have to add an unevidenced model of the changes we can't see.

That's not to say scientists don't or shouldn't speculate about other models. But extraordinary claims need extraordinary evidence. There is a significant burden of providing a mechanism by which any such model could be tested, let alone actually testing it.

 

[timmdeeg]

Can we / should we extrapolate that the rate of expansion of space, of a universe that is / may be infinite, is potentially infinite?

You seem to think of the rate of the expansion, i.e. of the Hubble Constant, as of some kind of velocity which increases in an accelerated expanding universe. This isn't the case. Instead in an accelerated expanding universe the HC is decreasing until in the far future it will expand exponentially. Then the HC will have a constant value. This can be seen by checking the Friedmann Equations.

 

[kimbyd]

Can you be more specific? As I understand it, there are no particular theoretical reasons from within GR itself to think that the value of the cosmological constant we actually measure is too small to be believed; and anthropic arguments (which are admittedly much more speculative) indicate that the value we observe is just small enough to allow us to exist.

The main line of argument I'm aware of that would indicate that the value of the cosmological constant is too small to be believed is that based on quantum field theory, which gives an answer (at least on a certain set of more or less reasonable assumptions) about 120 orders of magnitude larger than the value we observe. But the QFT line of argument itself is based on viewing the "cosmological constant" term in the EFE as not being due to any intrinsic property of spacetime itself (which is what the term "cosmological constant" normally implies), but due to the quantum vacuum having nonzero stress-energy (which would be more aptly described as a theoretical explanation for dark energy).

Rendered in dimensionless terms, the cosmological constant is an absurdly tiny number (around 10^-120). This is the usual justification.

Some of the justification for this specific choice of dimensionless units comes from quantum mechanics, but it's not really a definitive prediction of the theory (there's no unique way of calculating the vacuum energy in QFT).

 

[PeterDonis]

Rendered in dimensionless terms

In GR, the cosmological constant is not dimensionless. It has units of curvature (or energy density, depending on which units you want to use to write the field equation or the Lagrangian). There is no reason in GR to expect that energy density to have any particular value.

As you say, there are QFT-based arguments that can be used to justify a dimensionless number corresponding to the cosmological constant, and that number then turns out to be of order $10^{-120}$. Those are the same arguments I already referred to that, theoretically, give a number 120 orders of magnitude larger than the value we observe, i.e., in QFT our "naive" theoretical expectation is that the dimensionless number corresponding to "the energy density of the quantum vacuum" should be of order unity.

 

[PeroK]

The technical understanding of the debate into GR et al is yet beyond my first-year BSc Physics level but that hopefully will come to me.
Just one last layperson question then if I may please, for a first-year physics student's understanding, specifically defined by redshift.
Just basic redshift measurements have identified that galaxies at the perimeter of our visual universe are receding from the Milky Way at circa the speed of light.
Leaving aside the various debates for reasons, could we expect that a galaxy 20 x the perimeter of our visual universe would show a commensurate increase in redshift?
Thanks
Martyn

If you really want to take time out from your course syllabus, try this:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

 

[Martyn Arthur]

That's absolutely brilliant, thank you very much, lots of data in basic form.
I'm getting onto the OU second-year maths course as I go along, (bought the textbooks on Amazon) starting that second-year course formally in October 2022.
At age 70 I'm not working on a career, just trying to understand what there is to be understood, indeed trying to undrestand why that which cannot be understood, cannot be understood.
Bear with me please I am a 70-year-old junior.
Thanks
Martyn

 

[Buzz Bloom]

Hi @Martyn Arthur:

I confess I am not sure I understand your point of view in Post #1, but I think that you may have missed the role of the Hubble constant H_0. Ignoring gravity, two galaxies which are right now at a distance D from each other will be moving away from each other at a velocity of D x H_0. The value is

1/H_0 = 14,400,000,000 years.​

Also at 31,536,000 seconds in a year,

1/H_0 = 454,118,400,000,000,000 seconds.​

To have the two galaxies move away from each other at the speed of light

V = 299,792,458 m/s (meters per second).​

The distance between these two galaxies would then be

D = V/H_0 = 136,141,271,359,027,200,000,000,000 meters​

Now consider one light year (1 ly).

1 ly = 9,460,528,405,000,020 m (meters)​

Therefore (approximately)

D = 14,400,000,000 ly (light years).​

However, this is not the radius of the observable universe, R_ou.

R_ou= 46,500,000,000 light years.​

What is observed at (approximately) this distance are not galaxies, but the state of the universe at the age of the universe when it was about 300,000 years old. This state is known as the time of recombination. This is the time when protons and electrons which has previousy been apart have cooled down to a temperature (approximately) 1100 degrees K) when the protons and electrons join to form hydrogen atoms. The key to understanding is that the farther way we observe something the younger it is because of the elapsed time for what is observed to reach us traveling at the speed of light.

Regards,
Buzz

 

[Dan White]

Well, the point about the universe outside our observable region is that we cannot see or affect it and it cannot see or affect us. So there's a sense in which modelling it is pointless. However, when we assume that the universe is the same everywhere ("spatially isotropic and homogeneous") we get a result that looks very similar to what we see. Occam's razor suggests that we should assume that the rest of the universe that we can't see is the same, because to believe otherwise we'd have to add an unevidenced model of the changes we can't see.

That's not to say scientists don't or shouldn't speculate about other models. But extraordinary claims need extraordinary evidence. There is a significant burden of providing a mechanism by which any such model could be tested, let alone actually testing it.

I'm not a physicist (as will become obvious the more I post here) but I watch a lot of Discovery channel. lol.

Let's say we are looking at a galaxy at the edge of the observable universe (I know you won't find any there, but go with it). In that galaxy is someone looking back at us. We each have a sphere of 45B ly radius that is observable. The universe that is between us and is common to both of us is the same ("spatially isotropic and homogenous"). The remainder of our universe is unobservable to the other person, yet we know it also to be "the same." Why, then would the rest of his universe, unobservable to us, also not be the same?

 

[PeroK]

Why, then would the rest of his universe, unobservable to us, also not be the same?

Why not indeed? This assumption is called the cosmological principle:

https://en.wikipedia.org/wiki/Cosmological_principle

 

[Dan White]

Why not indeed? This assumption is called the cosmological principle:

https://en.wikipedia.org/wiki/Cosmological_principle

lol. I guess Isaac Newton got me by about 300 years, not that I really figured it out on my own, anyway.

 

[Orodruin]

It is important to underline that it is still an assumption even if it seems like a reasonable one.

 

[PeterDonis]

Let's say we are looking at a galaxy at the edge of the observable universe (I know you won't find any there,

Why wouldn't you find galaxies at the edge of our observable universe?

 

[Dan White]

Why wouldn't you find galaxies at the edge of our observable universe?

What I meant is that if you're looking that far away with a telescope then galaxies had not yet formed.

 

[phinds]

What I meant is that if you're looking that far away with a telescope then galaxies had not yet formed.

You really need to think this through. Having been formed and being visible are TOTALLY different things. Think about what a comoving observer at that location would see vs what we see of that location.

 

[Bandersnatch]

You really need to think this through.

There's nothing wrong with his thinking that I can see.

 

[PeterDonis]

What I meant is that if you're looking that far away with a telescope then galaxies had not yet formed.

Had not yet formed at the time the light we're seeing now was emitted, yes. But in the rest of your post you're not talking about an observer at that location at the time when that light was emitted. You're talking about some observer there "now", seeing light emitted from whatever existed at our galaxy's location then. And according to our best current model, what that observer would see "now" in light emitted then from our location would be similar to what we see now in light emitted then from their location.

 

[phinds]

There's nothing wrong with his thinking that I can see.

Well, the phrasing is ambiguous. He got what WE see right but a comoving observer would not see what he seems to think would be seen.

 

[Dan White]

Well, the phrasing is ambiguous. He got what WE see right but a comoving observer would not see what he seems to think would be seen.

Sorry for confusing everybody. I'm new here and only have a superficial knowledge of this stuff. I signed up here to learn more. I can see that one needs to be precise when commenting here. My original post was like a mixed metaphor. Let me restate so I know I understand correctly:

We can view light from the earliest time of the universe because of the distance light has to travel to get to us. There were no galaxies that far back in time so we won't see any, or maybe just proto galaxies. If we now consider present day, that same region is now populated with galaxies, although we won't see them because it takes so long for the light to get here. Also, due to expansion, whatever was in that region billions of years ago is now much farther away. In my original comment I was merely considering two observers, each in their own galaxy, at the edge of each other's observable universe.

 

[PeroK]

Sorry for confusing everybody. I'm new here and only have a superficial knowledge of this stuff. I signed up here to learn more. I can see that one needs to be precise when commenting here. My original post was like a mixed metaphor. Let me restate so I know I understand correctly:

We can view light from the earliest time of the universe because of the distance light has to travel to get to us. There were no galaxies that far back in time so we won't see any, or maybe just proto galaxies. If we now consider present day, that same region is now populated with galaxies, although we won't see them because it takes so long for the light to get here. Also, due to expansion, whatever was in that region billions of years ago is now much farther away. In my original comment I was merely considering two observers, each in their own galaxy, at the edge of each other's observable universe.

It's worth mentioning that the farthest back we can see using light is the CMBR (Cosmic Microwave Background Radiation). This is from the earliest time that the conditions in the universe allowed light to propagate effectively indefinitely. Prior to that the universe was so dense that light tended to get absorbed by collisions with elementary particles.

https://en.wikipedia.org/wiki/Cosmic_microwave_background

That said, reasearchers are now trying to detect gravitational waves from even earlier times, as an alternative means of studying the very early universe. Here's something on this from MIT.

https://news.mit.edu/2020/universe-first-gravitational-waves-1209

 

[Dan White]

It's worth mentioning that the farthest back we can see using light is the CMBR (Cosmic Microwave Background Radiation). This is from the earliest time that the conditions in the universe allowed light to propagate effectively indefinitely. Prior to that the universe was so dense that light tended to get absorbed by collisions with elementary particles.

https://en.wikipedia.org/wiki/Cosmic_microwave_background

That said, reasearchers are now trying to detect gravitational waves from even earlier times, as an alternative means of studying the very early universe. Here's something on this from MIT.

https://news.mit.edu/2020/universe-first-gravitational-waves-1209

Is there something specific about the CMBR that makes it stand out from other radiation that did not originate from the Big Bang? They say something like 1% of the static on your TV comes from the CMBR. How do we know?

 

[Ibix]

Is there something specific about the CMBR that makes it stand out from other radiation that did not originate from the Big Bang?

Yes and no. It's just microwave radiation and totally undistinguished in some senses. However, it's black body radiation at a very nearly constant temperature coming from all parts of the sky. So there's nothing special about the radiation, but the source and the spectrum are fairly distinctive.

They say something like 1% of the static on your TV comes from the CMBR. How do we know?

I'd take anything "they" say with a pinch of salt, unless it's backed up by some maths. A quick Google of the frequency of TV frequencies suggests that they are considerably below the peak frequency of the CMB (a few hundred MHz versus 160GHz), but there's some CMB power in the TV bands. I don't know about 1% though - I'd tend to think local interference sources vary wildly in strength so I'd be very suspicious of a blanket figure like that.

 

[Bandersnatch]

Isn't the old TV static mostly thermal emissions from the atmosphere? You'd then get that 1% from relative temperatures of the sources.

 

[Vanadium 50]

What is this "static" of which you speak?