EvC Forum: Motion in an expanding space

Sylas,

I don't really get it.

The rate of expansion 70 km/sec/MPsec is called the "Hubble constant". The term is misleading, because the Hubble constant is not in fact a constant at all! It is a constant value through all space, but it changes with time.

Got it. No problem. But.. WHY does this constant change with time?

But what happens as time passes? In a simple zero density case, a galaxy at 1 MegaParsec distance, which is receding with the Hubble flow at 70 km/sec, will continue to recede at that same rate, even as it moves further away. So, taking a MegaParsec as 3*10^19 km, then after 4.3*10^17 (BC:changed) seconds, the galaxy will be 2 MegaParsecs away. This is about 14 billion years.

(Does than number sound familiar? It should! It is about the age of the universe, since the time when galaxies are at zero separation distance.)

OK... what does this mean? Does it mean that space has expanded to 1Mps since the big bang? I guess not, since the Hubble constant is changing in time. So, what does this mean?

At that time, the Hubble flow rate will be only 35 km/sec/MPsec. In other words, the flow rate is inversely proportional to age.

I've no idea how you got here. Clearly the "Hubble flow rate" is half of what it was as time doubled. But I have no understanding of the mechanism of WHY this would be.

Now there are a few more quibbles. A "cosmological constant" tends to maintain the Hubble flow at a fixed rate. As galaxies move further away, therefore, the rate of recession increases. In the extreme case this is what happens in so-called "inflation". If we have an inflationary expansion, so that the Hubble constant remains at 70 km/sec/MPsec indefinitely, then your answer is correct. (I think! I've only been learning about this myself quite recently.)

Whoa! Crazy! Now I'm really lost! But tell me this--if this "cosmological constant" didn't exist, and the "Hubble flow rate" was changing over time, then we WOULD see space "pull on" the particles--i.e. the particles would be seen to "accelerate" (i.e. change their relative velocity) without having any force applied to them.

On the other hand, gravity from mass in the universe tends to retard expansion.

Back in the crap hole. What?? How can gravity retard the expansion of space?? I can understand how it retards the expansion of mass, but .. space? How the heck does that work?

At present, there seems to be a small cosmological constant, but not enough for inflation, and also a small mass density which acts to retard expansion rates. So to a first approximation, we can just use the zero density model. Assume that the expansion of space is such that objects moving with the flow of expansion continue to recede at the same velocity as they move further away.

Shoot, lost again. I thought you said our universe was an inflationary universe (due to the cosmological constant). But then you say the cosmological constant is not enough for inflation. Which is it? Am I misreading?

Sorry for all the questions. i'll appreciate any comment that you make. I'll stop here for now, so that I avoid dousing you with further questions (i.e. the questions are raining down on you?)

Thanks,
Ben

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