Age of Universe: 13.7B Years & Cosmologist Qs

[Jvonderlinn]

I am not a cosmologist so do not understand how light from the most distant galaxies could take 13.7 billion years to reach us when 13.7 billion years ago the universe was much smaller.

 

[marcus]

I am not a cosmologist so do not understand how light from the most distant galaxies could take 13.7 billion years to reach us when 13.7 billion years ago the universe was much smaller.

There is a simple explanation. Relativity does not forbid distances (outside our local neighborhood in space) from increasing faster than c.

So a photon which starts out coming in our direction can at first "lose ground" and not make any progress. It can spend a long time "trying" to come to us without getting any closer, because of the ability of distances to increase faster than c.

The "charley" link in my signature has a good explanation of how the photon can eventually get to us (because of the declining expansion rate), you might take a look. The first page is blank so scroll down.

In standard model cosmology most distances (i.e. to most of the objects we see today) have been increasing faster than c, for most of the history of expansion. It is, in that sense, typical. Just how things are.

Geometry of space is "live", dynamical. That is what spacetime curvature means.
Fortunately the percentage change is very small. Currently only about 1/140 of one percent every million years. That kind of change in a small distance would not be detectable. But for a very long distance it can amount to quite a speedy expansion. And in the early universe the percentage rate was far larger than that.

Perhaps what I've said is puzzling. All I can suggest is that you keep on asking questions.

 

[marcus]

Jvonderlinn,
since you are new, let us forget that "charley" link in my signature, which is to a more advanced paper outside Physicsforums.

First learn how to use Physicsforums. If you want to reply, one simple way to do it is to look down at the bottom on the left where it says NEW REPLY. Click that button, get a box to type in. Type a message. Then click SUBMIT REPLY.

There are also other ways to reply----with the QUOTE button, and also with the small "quick reply" message box at the bottom of the page. You can type in that message box and then press "post quick reply".

If you are a beginner you should practice using these.

If you have any difficulty, you can write a PM (private message) and ask for help.

Good luck.

 

[Orion1]

There is a simple explanation. Relativity does not forbid distances (outside our local neighborhood in space) from increasing faster than c.

Does General Relativity forbid distances (outside our local neighborhood in space) from decreasing faster than c?

 

[qraal]

Does General Relativity forbid distances (outside our local neighborhood in space) from decreasing faster than c?

Collapse can be as quick as expansion. GR doesn't violate SR - instead the speed of light is measured everywhere the same, so long as the distances involved are small compared to the curvature of space-time. Bigger distances mean curvature effects will change the results.

 

[attentater]

can i get to know why our ears close when we are traveling in an aeroplane??

 

[JaredJames]

can i get to know why our ears close when we are traveling in an aeroplane??

Your ears don't close. They 'pop' due to the pressure change. An effect that can also be noted traveling to different heights perhaps via hills / mountains etc.

Next time, start your own thread. Don't hijack someone elses.

 

[Jvonderlinn]

Thanks Marcus. Problem is my browser does not give me the reply option, only Preview Post and Submit Reply, neither of which present a text box. Not sure how I found the one I am now using? I tried Chrome and IE and same problem with each. Anyhow, I did get the Charley link to work and have read a bit of the paper-fascinating. But the mystery deepens. Since the most distant galaxies are beyond the Hubble distance and are receding faster than c, how does their light ever reach us?

 

[marcus]

Collapse can be as quick as expansion. GR doesn't violate SR - instead the speed of light is measured everywhere the same, so long as the distances involved are small compared to the curvature of space-time. Bigger distances mean curvature effects will change the results.

graal, you have been making great contributions. I'm busy now. Could you reply some to von der Linn, who is a newcomer? Describe how light can get to us even if it is emitted by a galaxy which is receding > c? Would very much appreciate.

 

[marcus]

... Since the most distant galaxies are beyond the Hubble distance and are receding faster than c, how does their light ever reach us?

I just got back and have a minute, so I'll respond

You are right that if a galaxy is further than the Hubble distance then the distance to it is increasing > c.

The Hubble distance is c/H(t) so if H(t) decreases, the distance c/H increases.

A photon from a galaxy outside can make it inside the Hubble sphere even though it is losing ground, if the Hubble distance reaches out to it.

So the photon can keep trying, and hoping, to get to us, and even if it is losing ground it is still closer than the galaxy it came from---and the c/H distances may increase fast enough to eventually reach it. Then the photon is OK and will make it.
---------------------------

I don't know how far along in school you are. If you have had algebra you might be interested to know that H(t) is by definition a'(t)/a(t).
The time derivative of the scale factor divided by the scale factor. You can think of it as the fractional (or "percentage") rate of increase of the scale factor.

Even if a'(t) is increasing ("accelerated expansion") it can still be that a(t) is growing even faster, so that the ratio a'/a is decreasing. And this is how it is in fact. Even though we have a slight bit of acceleration, H(t) is still decreasing and will continue to decrease. And so the distance c/H will continue to grow (towards some asymptototic limit of around 15 or 16 billion LY.)

Ask questions about the scale factor if it is not familiar to you and if you want to know.

Maybe some of the others will answer. We have smart helpful people around nowadays.

 

[bapowell]

If you have had algebra you might be interested to know that H(t) is by definition a'(t)/a(t).

I didn't know they taught you time derivatives in algebra, Marcus

 

[Andrew Mason]

I see my April fool's post has been deleted by our mentor. No sense of humour here?

AM

 

[marcus]

I didn't know they taught you time derivatives in algebra, Marcus

My feeling is this J von something is quite smart and might just KNOW about time derivatives.
Algebra is where people get over being scared of notation, so I think J is OK. And he/she may actually be a grownup, not a high schooler. I have no clues or even very much in the way of guesses.

 

[Jvonderlinn]

I always thought I was quite smart, but very lazy! I have a BS in Aero engineering and worked at Boeing for 37 years. I used to understand time derivatives but just turned 75 and forget stuff as fast as I read it. Re your wonderful answer, "I'm beginning to see the light". Love reading about Cosmology and just finished Hawking's latest book. Convenient that he thinks the total energy of the universe is zero.
Thanks for your explanation, you must be a great teacher.

 

[marcus]

Thanks, I did teach some (and liked it). I'm retired, about your age give or take.

If you are not too busy I'd suggest

google "wright balloon model" for short animation to watch and also google
"wright cosmo calculator" learn to think more with redshift and derive distance and lookback time from the redshift (since that is the measured quantity)

there is also Siobhan Morgan's "cosmos calculator" where she makes you type in 3 parameters first (.27, .73, 71) for matterfraction, cosm. const., and Hubble rate, before you can use it but then it works like Wright's "cosmo calculator".

And eventually I'd suggest learning a little about quantum cosmology, where you resolve the BB singularity. Some QC models predict features of the microwave background which makes them vulnerable to testing. So that is potentially an exciting development.

I'd suggest learning to use the Stanford SLAC database called "Spires". Use the German mirror because it is faster.

You might want to save this link in case you ever get into QC:
http://www-library.desy.de/spires/hep/

and here are the QC papers 2009 and later, ranked by citation count:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2008&FORMAT=www&SEQUENCE=citecount%28d%29

here is a recent (2010) review article by one of the main figures in the field, Abhay Ashtekar

"The Big Bang and the Quantum" (a popular-sounding title, but he is a solid expert and it is written for cosmologists)
http://arXiv.org/abs/1005.5491
Click on PDF to get the complete text.

 

[Chronos]

Hawkings zero energy concept is indeed fascinating. It is a way of permitting the existence of the universe without violating the laws of thermodynamics. Lawrence Krauss gives an entertaining talk on this here: