# Hubbles law and expansion of the universe

by trot
Tags: hubbles law
 P: 6 Hi, i am confused and perhaps someone can help me:(haha i am so confused i need someone with atleast a phd or doctorate in physics ) v/d=H0 v= velocity of distant galaxy H0= hubbles constant d=distance to galaxy The problem i having is that if since the expansion of the universe is at present accelerating, the value of the Hubble constant is presently increasing, the Hubble constant must/has always been decreasing because the universe is getting older and 1/H0 = age of universe can someone tell me whether the hubbles constant is increasing or decreasing, and why. At the big bang is hubbles constant very small, or very large? Is density proportional to the hubbles constant? or what is the link between them, i guess that at big bang density is very large, which means very large gravational forces, rate of expansion is therefore slow, then as the universe expands and spreads out, expansion gets faster(density smaller) H0 is therefore bigger as time goes on, this is another contradiction to the earlier piont about, 1/H0 = age, Is the Hubbles constant the exact same as the rate of expansion? If the rate of expansion was constant, then as time goes by galaxies move further away and from hubbles law are moving faster and faster, so the rate of expansion is not constant its getting faster? someone explain please, you can see how confused i am. Any help is appreciated! Thank you!
 PF Patron P: 1,261 Hubbles constant is time Its better thought of as Hubble rate In the cosmology forum Marcus just posted a recession velocity chart showing the rates of expansion. Its the last post in the look 88 billion years and see the Universe sticky thread
PF Patron
P: 22,332
 Quote by trot ... The problem i having is that if since the expansion of the universe is at present accelerating, the value of the Hubble constant is presently increasing, the Hubble constant must/has always been decreasing because the universe is getting older and 1/H0 = age of universe ...
It is not true that Hubble parameter is increasing. When people use the word "acceleration" they are talking about something else.

It is also not true that the age of the universe = 1/Ho

They happen to be CLOSE probably by coincidence, but they are different concepts.
According to the Planck mission data that was just released 1/Ho = 14.4 billion years.
The age of universe (again using Planck mission data) is more like 13.8 billion years. Substantially different

Some of your puzzlement seems due to misinformation. Believing things that are not true. Or misinterpreting what you have heard like about "acceleration". People here will think how best to help. I'll try. There is an FAQ. But also it may work if you just ask simple questions, patiently.

PF Patron
P: 22,332

## Hubbles law and expansion of the universe

 Quote by trot If the rate of expansion was constant, then as time goes by galaxies move further away and from hubbles law are moving faster and faster, so the rate of expansion is not constant its getting faster? someone explain please, you can see how confused i am. ...
Part of the trouble is with the English language and the word "rate". It is used in two different senses: as percentage growth rate, or as gross speed of increase. I have to give an example.

Suppose you put \$100 in a special bank account where the bank starts out paying 5% interest but each year it lowers the interest rate by a tenth of a %. So the next year it pays 4.9% and the next year it pays 4.8%.
This goes on for 10 years until it is paying only 4%, and then it continues at that level from then on.

Is your money growing at a DECREASING rate? (because the interest rate keeps declining)
Or is it growing at an increasing rate? (because the dollar amount gained by your account keeps getting bigger.)

100*1.05 = 105 (you gained 5)
100*1.05*1.049 = 110.145 (you gained more than 5 this time)
100*1.05*1.049*1.048 = 115.43196 (you gained more than last year)
100*1.05*1.049*1.048*1.047 = 120.857262 (you gained still more)

Hubble rate is like a percentage growth rate, or bank interest rate, and it has been declining since early days and is expected to keep on declining---but so slowly that if you pick two widely separated galaxies and record the distance it will STILL be growing by an increasing amount.

Percentagewise less and less, but in terms of raw distance additions more and more.
 PF Patron P: 1,261 There has been a couple related threads on this subject. http://www.physicsforums.com/showthread.php?t=685265 has lots lots useful info this site has a decent article though youll to scroll down to the Hubble section http://www.physics.fsu.edu/users/ProsperH/AST3033/Cosmology.htm
 PF Patron Sci Advisor P: 22,332 Mordy probably has some good links. I have to go out don't have time to say much. Hubble rate now amounts to 1/140% per million years expected to decline to 1/165% in longterm future and level out there (that is according to best data as of 2010, socalled WMAP plus other stuff) And more recently Planck mission gave revised numbers which amount to present distance growth rate 1/144% per million years expected to decline for many billions of years and likewise level off, at estimated 1/173% per million years. Like the bank paying a declining rate of interest.
 Mentor P: 5,182 To add to what others have said, you can easily see that, "the universe expansion rate is increasing" and "the Hubble parameter is increasing" are NOT equivalent statements, and the second one is NOT required in order for the former one to be true. Suppose that the Hubble parameter were actually constant at a value of H0. Suppose two galaxies are separated by distance d. Now, this means that they recede away from each other at speed v = H0d. But if they're moving away from each other, that means the distance between them gets even larger (d increases). This means that their rate of recession v, also increases (remember that H0 is constant, so a larger d implies a larger v). But if they recede from each other even faster, then d becomes even larger still. But if their distance is even larger, then v will become even faster…positive feedback! This is exponential growth, which is a dramatically accelerating universe. For something less dramatic, like constant acceleration, or a constant expansion rate, or a decelerating universe, it is actually necessary that H declines with time.
Mentor
P: 8,262
 Quote by trot H0= hubbles constant
 Quote by Mordred Hubbles constant is time Its better thought of as Hubble rate
Just to clarify a bit of notation, the Hubble rate is indeed not a constant (i.e. H=H(t)). However, when cosmologists use a subscript "zero", it means "the value of that parameter today", so H_0 is often called the Hubble constant, but really it is "the value of the Hubble rate today".
 PF Patron P: 1,261 Good points when I mentioned it as Hubble rate I should have mentioned the correct name is Hubble constant. The name is more a historical one http://www.physicsforums.com/showthread.php?t=683384 This discusses that Hubble parameter would probably be a more accurate term however as its commonly taught as Hubble constant its good to be aware that it isn't really a constant despite the name. As it is time dependant.

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