# Question on Hubble's Law

1. Jul 10, 2012

### khurramc

The Hubble's Law is based on empirical evidence that those objects further from us are moving away from us proportionate to their distance from us.

My question is that the further we look back the further we look back in time. And so, why is that when we see further, and notice objects moving away faster, do we state that the universe is expanding?

I mean even in a universe that was not expanding, wouldn't objects back in time (and therefore further from us) be moving faster than objects nearer to us just because of the speed of light and the fact that our view further into the universe is but a picture of how the universe was in the past?

I understand that objects further away from us are moving away faster. I am just concerned with the fact, that would that not be natural if you were looking back in time to an explosion slowing down and a universe not expanding? Since even if expansion was slowing down, we would notice that objects further way would depict speeds of expansion in the past, while objects closer would reflect speeds closer to the present. Therefore, if the the expansion was slowing down, wouldn't objects further depict a faster speed than objects closer?

Any explanations to my query would be much appreciated.
Regards
Khurram Chaudhry

2. Jul 10, 2012

### Staff: Mentor

If the universe would not expand, these objects would not move away at all, apart from random velocities due to gravitational influence from objects nearby.

An explosion in space violates the observation that the universe looks the same everywhere, together with some more technical details. It is an expansion of space itself, not an expansion of matter in space.

The chance in the expansion rate (which is not the same as the expansion rate!) is a different issue. Compared to a constant expansion rate, you are right that objects far away are not as fast as you would expect. However, space was always expanding, so they are still more redshifted than objects closer to us.

3. Jul 11, 2012

### Chalnoth

A more accurate way of talking about the redshift is that when the universe expands, the wavelength of photons is expanded as well. So if we observe an object and the photons from that object have wavelengths that have doubled, then we know that the universe has expanded by a factor of two in the interim since that light was first emitted. The universe has expanded more for further-away objects, and so we see further-away objects as having higher redshifts.

A less accurate but possibly more intuitive way of thinking about it is to just ignore the weirdness of General Relativity and consider redshift to be a measure of velocity of the far-away object. In an expanding universe, the recession velocity of an object is given by the Hubble expansion rate times that object's distance (i.e. $v = Hd$). So having a higher recession velocity (measured via redshift) in the past doesn't necessarily mean that H was higher in the past: in fact, H could, in principle, be lower and we'd still see further-away objects having higher velocities (as long as the reduction in H is smaller than the increase in d). In practice, though, H has decreased over time for the duration of our observable universe's existence.

4. Jul 12, 2012

### Naty1

That's logical, as far as it goes, and I would not be surprised if some scientists perhaps started out from that perspective....but as noted above, we have observation evidence that is contrary to those assumptions....so scientists had to develop a different model that matches observations. That model is the FLRW model with lambda CDM parameters.