# Why do people think red shift is caused by big bang?

1. May 8, 2012

Question: as title says.
I'm not a physicist at all and I do not know much about how people obtained the big bang theory from red shift. So here is what I thought from red shift:

In a time period t1, a distant body emits a light wave with a certain amount of energy with n oscilations(I dont know whats it called!). In the big bang theory, it is caused by the increase of space so the wavelength increased. However in Einsteins... some relativity theory (special?), if an object moves faster its time relative to the space will increase; So if the time of observer increases to t2 but light wave with same energy reaches the observer, its wavelength will also increase. So could that be, that the space is not expanding but the observer, whether its the earth, solar system or the milky way's speed increasing?
t1 < t2
λ1 = t1/n
λ2 = t2/n
λ1 < λ2

Victor Lu, 16
BHS, CHCH, NZ

2. May 8, 2012

### Vagn

If that was the case only half the universe would be red shifted, the half in the direction the Milky Way was moving would be blue shifted, which isn't seen. Every galaxy outside of our local supercluster is red shifted. So we conclude that the universe is expanding to explain why the red shift is in all directions.

3. May 8, 2012

### Drakkith

Staff Emeritus
The standard theory of cosmology, aka the Big Bang theory, is underpinned by General Relativity. GR says that one of the ways that the universe can expand is by the geometry of space itself causing this expansion. What this does is increase the size of a volume of space over time. The development of the Big Bang theory and why we believe that it is the expansion of space that causes this redshift instead of normal doppler shift is enormously complex and there isn't much hope of explaining it very well unless you already understand some pretty complicated math that you wouldn't take until midway through college. But trust that the available evidence points towards the BBT as being the most accurate description of the Universe we currently have.

4. May 9, 2012

### GTOM

I read a theory, that tryed to explain the red shift with gravity instead of speed, does it make some sense?

5. May 9, 2012

### Drakkith

Staff Emeritus
Both can cause redshift.

6. May 10, 2012

### GTOM

So, how can be so sure, that we see distant galaxies in red because of receeding, and not gravity?
/Well, i asked the maker of that theory, why gravity blueshift doesnt compensate for gravity redshift, i havent understood his explanation very well. He spoke about the average density of space. Maybe the missing dark matter indicates that we have false assumptions about our galaxy's mass?/

7. May 10, 2012

### Ich

If it were gravity, we'd see blueshift instead. Further, there'd be no linear Hubble law.

8. May 10, 2012

### GTOM

If i imagine space like a giant orb, in the center, the gravity fields neutralize each other, while on the border, there is big gravity.
So if light comes from the border into the center, and i think those distant galaxies are on the border, then it comes from stronger gravity to lower gravity, and light is redshifted when leaving the gravity well.

Or where am i wrong?

9. May 10, 2012

### Vagn

That would require the local supercluster to be at the centre of the universe, which would be against the Copernican Principle, which if violated tends to lead towards philosophical/religious discussions and as such isn't really relevant to a discussion on cosmology. The universe also appears to be flat, or at least very close to it, so there's no reason to suppose it would necessarily be spherical.

10. May 10, 2012

### mesa

11. May 10, 2012

### Drakkith

Staff Emeritus
You're making a couple of very big assumptions.

A. That the Universe has a border.

B. That we are at or near the center of said universe.

In addition, a few of your ideas are not correct. First, light entering from such a border would be traveling through an area of homogenous matter and would experience no redshift, as the gravity from all sides will cancel out. There is no gravity well as long as you're inside the border. (On the large scale. Obviously small scale effects would cause red/blue shift, but over cosmological distances these cancel out.)

Second, the gravity from such a border would not be more than the gravity near the center.

12. May 10, 2012

### mesa

I asked my physics professor a couple weeks ago this very question and he was quite adamant that gravity does not cancel out with systems containing multiple point masses; unlike the behavior of other forces such as electromagnetism.

13. May 10, 2012

### Drakkith

Staff Emeritus
Let's be clear. The NET force from gravity will cancel out. However the curvature caused by mass will not. Adding more mass to an area of space will cause more curvature.
Now that I think about it, I am actually not certain how light would be affected if it was emitted at the boundary of a large homogenous area of space. I want to say that it wouldn't be blueshifted on the way to the center, but I'm not 100% sure on that.

14. May 10, 2012

### mesa

So gravity between two independent systems can cancel but a conglomeration of mass will not? Am I getting this right? That just seems a little odd because wouldn't the individual atoms in the mass also cancel? Or am I just looking at this completely wrong lol?

If so, where would be a good area to go to get more information on how this works so I don't clog up this thread any more than I already have?

15. May 11, 2012

### Ich

There are some misconceptions.
If we were at the center of a homogeneous massive sphere, everything, including light, would accelerate towards us. That definitely means blueshift.
The funny thing is: you don't need a finite sphere. It works the same way if the universe is infinite, as the gravitation of the matter outside the sphere cancels out (shperical symmetry assumed).
Further, the blueshift grows - to first relevant order - quadratically with distance, not linearly. Even if we had "repulsive gravity" like Dark Energy producing redshift instead (which it in fact does on larger scales), it would never produce the observed Hubble's law.

16. May 11, 2012

### Drakkith

Staff Emeritus
I thought there would be no blueshift, can you elaborate?

17. May 11, 2012

### Ich

If you drilled a hole through the earth and stepped over it, you'd certainly fall down. Same thing with light, it gets blueshifted in that direction (it can't accelerate). That means the potential is lowest at the center, and infalling things have there the highest energy. See fig. 8 here.

Gravity cancels at the center in the sense that you'd be weightless there (and only there). Still, having fallen through regions where you were not weightless, you have gained significant energy. Blueshift - and kinetic energy - are defined by the difference in potential, not acceleration.

18. May 11, 2012

### mesa

I understand. So gravity really isn't "cancelled out" it would just feel that way from that perspective since you would be pulled by gravity in all directions at the same time.

Drakkith, I am guessing this is what you were so patiently trying to explain to me lol?

19. May 11, 2012

### Drakkith

Staff Emeritus
No, I was apparently mistaken. I thought that upon entering a solid uniform sphere, you cease to accelerate further.

20. May 12, 2012

### mesa

Interesting, but if you were 'moved' from the center and even though you would be further from the opposite side (with respect to the one you are headed to) wouldn't there be more mass to compensate for the distance meaning possibly still no gravitational effects?

Ah okay,