How big Is the universe?

[900SL]

Was cosmic inflation faster than the speed of light?

[vlaiv]

20 minutes ago, 900SL said:

Was cosmic inflation faster than the speed of light?

I'm certain "it was", but I'm not sure two are comparable in speed.

Speed of light is measured as distance over time.

Cosmic inflation is measured (if I'm not mistaken) as scale factor over time.

 

[andrew s]

12 minutes ago, vlaiv said:

I'm certain "it was", but I'm not sure two are comparable in speed.

Speed of light is measured as distance over time.

Cosmic inflation is measured (if I'm not mistaken) as scale factor over time.

 

Yes many times but as @vlaiv said they are not comparable. This is true of both the proposed initial "inflation" and the early stages of the currently observed Hubble expansion. 

Even now if you go far enough away it is expanding faster than c. Remarkably we can see things that are receding at about 3c. 😊

Regards Andrew 

Edited July 22, 2023 by andrew s

[ollypenrice]

When we think of the expanding universe we think of Hubble's observation of the linear relationship between recession velocity and distance to galaxies. We then think of the fact that observers in any of these galaxies will see exactly the same thing under the 'expansion of the universe' hypothesis.

It seems to me, therefore, that every observer in every galaxy feels that they are at rest and will be tempted to attribute any 'motion' to all the others.  None of these observers considers themslelf to be accelerated. It also seems to me that they are right in this view since, if each observer were being accelerated, in what direction would they be accelerated? If they were being accelerated they would have to be accelerated in one direction but not all the others, so violating their equivalent of Hubble's observation. The notion of the expansion of space allows them to remain at rest and yet move away from each other. This allows them to move away from each other at superluminal velocities.

Now, I'm not daft enough to imagine that I have this right but I'd like to know, from those with a proper understanding, what is wrong with my thinking.

Olly

 

[andrew s]

1 hour ago, ollypenrice said:

Now, I'm not daft enough to imagine that I have this right but I'd like to know, from those with a proper understanding, what is wrong with my thinking.

Seems fair enough to me. 

You can tell if your moving with respect to the CMB by the red / blue shift dipole caused by the motion. It is removed from the CMB images normally shown.

Regards Andrew 

Edited July 22, 2023 by andrew s

[saac]

57 minutes ago, ollypenrice said:

When we think of the expanding universe we think of Hubble's observation of the linear relationship between recession velocity and distance to galaxies. We then think of the fact that observers in any of these galaxies will see exactly the same thing under the 'expansion of the universe' hypothesis.

It seems to me, therefore, that every observer in every galaxy feels that they are at rest and will be tempted to attribute any 'motion' to all the others.  None of these observers considers themslelf to be accelerated. It also seems to me that they are right in this view since, if each observer were being accelerated, in what direction would they be accelerated? If they were being accelerated they would have to be accelerated in one direction but not all the others, so violating their equivalent of Hubble's observation. The notion of the expansion of space allows them to remain at rest and yet move away from each other. This allows them to move away from each other at superluminal velocities.

Now, I'm not daft enough to imagine that I have this right but I'd like to know, from those with a proper understanding, what is wrong with my thinking.

Olly

 

My understanding is that the galaxies are not moving through space itself due to the expansion of space rather they move with the space itself. We would need to separate out these two different motions, the local motion caused by gravitational influences (andromeda and Milky Way for example)  and then the metrical expansion motion.  Now here I'm going to go out on a limb; I would have thought that as the metrical expansion is happening in all directions then so too is the acceleration - the acceleration of any galaxy (metrical acceleration)  would appear the same to any observer as it is caused by expansion of space itself. I do however wonder if the expansion is truly uniform or if it influenced by any non uniformity in the distribution of whatever is causing the expansion. Thinking about it is that not what we see in the irregularities in the CMBR- don't know?

Jim 

Edited July 22, 2023 by saac

[andrew s]

12 minutes ago, saac said:

I do however wonder if the expansion is truly uniform or if it influenced by any non uniformity in the distribution of whatever is causing the expansion.

This effect has been proposed as a solution to the Hubble tension between "local" and CMB derived value.  Any very large scale significant non uniformity in the mass energy distribution will impact the local metrical expansion. 

Regards Andrew 

Edited July 22, 2023 by andrew s

[ollypenrice]

14 hours ago, saac said:

My understanding is that the galaxies are not moving through space itself due to the expansion of space rather they move with the space itself. 

Jim 

This is how I'm tempted to think of it as well. What gives me pause, though, is the thought that the idea requires space itself to be able to define a location and I thought that relativity prohibited this, insisting that position can only defined via a relationship between different objects.

Olly

Edited July 23, 2023 by ollypenrice
False click

[andrew s]

17 minutes ago, ollypenrice said:

This is how I'm tempted to think of it as well. What gives me pause, though, is the thought that the idea requires space itself to be able to define a location and I thought that relativity prohibited this, insisting that position can only defined via a relationship between different objects.

Olly

As I mentioned above, the CMB does provide a reference that can be used as an almost "universal " reference for local, on a cosmological scale, motion.

At very large distances and due to the dynamics of spacetime comparing relative positions, velocities etc. is very problematic and very model dependent. 

As a simple example of the effects of spatial curvature.  Consider two cars on opposit sides of the equator heading north at a constant speed v. They are initially driving parallel to each other with zero relative velocity.  However,  by the time they reach the poles the are in for a head on collision at a relative speed of 2v.

Physics is fun.

Regards Andrew 

Edited July 23, 2023 by andrew s

[dan_adi]

On 17/04/2023 at 14:51, VirgoCluster25 said:

Google has told me that the universe is 93 billion lightyears in diameter and I was also taught it was expanding at the speed of light. But I was also taught it was 13.7 billion years old meaning it must be at least around 27 billion light years in diameter so is you universe 93 billion years old  or is it 27 billion light years in diameter. What do you guys think?  

Cosmology_calculator · Streamlit (go.ro)

Enter a redshift in the calculator and it will compute everything. The maximum redshift is 1089, this takes you very close to the big bang. Look at the numbers and the space time plot. The plot was easier for me to understand than raw numbers, although it was a total headache to code the graph, namely the light path

So, if you enter a cosmological redshift (z) of 1089:

• the universes age at that z was 0
• the look back time or light travel time was 13.78 billion years
• during the light travel time , the universe expanded very fast. Space can expand at a speed greater than the speed of light, the objects within space are confined to c, space can do what it wants.
• the proper distance today to the edge is 45.2 billion light-years. Multiply this by 2 to get the diameter ~ 90 billion light-years
• !! This creates some confusion ---> For objects in our cosmic backyard you can safely say 'if a star is 4 light years away that means the light from it travelled 4 years at c', but for objects far far away (bigger z) this statement no longer is true! The lookback time is time, not distance

The plot is more interesting than the numbers. You can see different horizons and the light path through space-time.

If you know the z of a galaxy you can enter the value and analyse its evolution through space-time. You can even see when or if it crossed the event horizon :). If it crossed the event horizon, the light that it emits now will never reach us again!, and it will slowly fade in our telescopes until sometime in the future, disappear.

 

Edited August 11, 2023 by dan_adi