Thread: big bang question

I only did high school physics and I don't remember much about that so I might not have my facts straight.

I was watching a documentary about the Big Bang and it said the expansion of the universe is known because of measuring the doppler shift in light.
So that would mean that the objects emitting the light, stars for example, are moving away from us.
The Universe is defined as everything.
So if we are in the universe, aren't we also expanding as well?
If we are also expanding shouldn't the distance that the light travels through remain in the same proportion to us and the light emitting object, and therefore there should be no doppler shift?
If the distant star is moving away from us and increasing the distance between it and us then we aren't expanding at all.
And so if it is only the boundary of the Universe that is expanding, and not the matter within it, what is it expanding into?

I probably have simplified one of these concepts too much. If anyone knows a solution to this problem I'd appreciate the relief from scratching my head about this.

Yep, I sometimes think the expansion of the universe idea is breaking the cosmological principle a bit by assuming the expansion does not affect us.
Matter is 99.99% empty space why should the metric expansion of space not affect it as well?

Originally Posted by PetTastic

Yep, I sometimes think the expansion of the universe idea is breaking the cosmological principle a bit by assuming the expansion does not affect us.
Matter is 99.99% empty space why should the metric expansion of space not affect it as well?

Matter is also 100% electric, magnetic and gravitational forces, while empty space is not. These forces keep solid bodies confined. And even if they wouldn't, you would still see redshift increasing with distance. Redshift depends on the relative velocity, not the rate of expansion. The expansion rate is the same everywhere, but the relative velocity increases with distance.

Example:
Two points in space have a distance of 100 kpc and their relative velocity due to expansion is 7 km/s. Let's assume another point at another distance of 100 kpc has the same relative velocity to the middle point of 7 km/s. However, relative to the point on the opposite side, its velocity is 14 km/s. So, the resulting redshift is higher.



The redshifts of p1 and p3 seen from p2 are the same. But as seen from p1, p3 has a higher redshift than p2.

Over the distance between the Earth and Moon, expansion would be about an atom's width per second, which local attraction can easily ignore.

On a bigger scale, our galaxy holds to it dwarf galaxies some four million light years away:


http://en.wikipedia.org/wiki/Local_Group


See Component Galaxies map.