# Expanding univese and light

1. Apr 18, 2012

### allo4

Ok I have very little knowledge of all this but I was thinking about the expansion of the universe and light. First off the universe is expanding from every direction right so there is no centre but then is it andromeda that is supposed to be coming towards us. Anyway I was going to use that in my argument but I'm sure someone will have a good explanation for that, what I was actually thinking was what if there was a centre and an edge but our visible universe is so small in comparison that we can't see far enough to se the difference in expansion closest to the centre in comparison than to the edge.
My thought about light then was that nothing can go faster and so this would be a likly candidate for the speed of the expansion at the moment I have more thoughts but these are probably stupid enough to start with...

2. Apr 18, 2012

### Staff: Mentor

Well, first off, welcome to PF. Second, don't beat yourself up over not knowing about expansion and the speed of light. We all start somewhere.

Currently, as you know, the universe is believed to be expanding without a center or edge. Whether there is or isn't is unknown, but we do have good reason for believing that there isn't either one. For one thing, expanding from a center would NOT cause galaxies to recede from us in all directions equally. The largest recession velocities would only be in the radial direction. (Radial is the direction from the center, like spokes on a bicycle tire)

Now, you may have heard that nothing can go faster than the speed of light. This is both true AND false. The most accurate statement is that nothing can outrun a beam of light. (AKA a photon) However, the expansion of space CAN and DOES cause objects to recede from us (and us from them) at a velocity that is greater than c. (c is the speed of light)

How is this possible you ask? The key is the expansion of space. You can imagine that galaxies and other objects are "carried" with space as it expands. This is helpful to help you visualize, but isn't quite accurate. Still, I recommend thinking of it that way.

All this is actually incorporated into General Relativity, whereas the maximum speed being c is only in Special Relativity. GR is the "big" theory that incorporates everything together, while SR simply works with non-accelerating observers and the absence of forces and was developed prior to Einstein working out GR. So SR is the precursor to GR.

3. Apr 18, 2012

### Mark M

Hey Allo, welcome to PF!

Firstly, Andromeda is very close to use compared to other galaxies. Expansion is very weak, the gravitational attraction between the two galaxies is much stronger. All other galaxies outside of our local cluster are receding from us.

Remember, because of the Cosmic Microwave Background, we know the universe is homogeneous. The CMB is essentially the light that we view at the edge of our observable universe (our light-cone) that depicts the universe 300,000 years after the big bang - and it describes a universe filled with a uniform soup of particles, eventually giving way to a universe with a uniform distribution of galaxies on very large scales.

It is important to keep in my that if the universe is finite, it wraps back around on itself, there is no boundary. Think of the surface of a sphere, or the surface of a torus.

Remember, nothing can travel through spacetime faster than light. Space itself is not restricted by this limit, spatial expansion can be far larger. The metric expansion of the universe is expressed as the Hubble Constant, which is a scalar that allows you to calculate the recessional velocity of a galaxy based off of its proper distance away from you. The Hubble Constant has been estimated to be approximately $70.8 \pm 4 (km/s)Mpc$ by WMAP.

4. Apr 19, 2012

### allo4

ok I was hoping that there was a connection between light and expansion for a lot of follow up questions but one (still ridiculous probably) is could it be that a mass doesn't warp the fabric of space but the other way around, the distortion of space creates a mass and a wave in space creates light.

5. Apr 19, 2012

### GeorgeDishman

Since the expansion is uniform (on average) throughout space, it doesn't make sense to describe it by a speed. Imagine putting a map of distant galaxies into a photocopier set to magnify by 1%. The rate of expansion of the universe is roughly 1% per 200 million years at present.

6. Apr 19, 2012

### Staff: Mentor

Mass is a property of matter and energy, not space. If space curvature caused mass then you would have to come up with some really weird rules to explain why it follows matter and energy around. It just doesn't make sense that way. Also, light is an electromagnetic wave that propagates through space. This was shown to be so over 100 years ago.

7. Apr 19, 2012

### GeorgeDishman

Slightly off-topic but can you explain what distinction you are making between "matter" and "energy"?

8. Apr 19, 2012

### Staff: Mentor

Matter: Protons, neutrons, electrons, quarks, hadrons, leptons, etc.
Energy: That ability to do work. Along with the more asbtract definitions.

Does that help?

9. Apr 20, 2012

### GeorgeDishman

Yes thanks. Having done more on cosmology lately, I tend to think of "matter" versus "radiation" as being a classification of types of energy.