Understanding through Discussion


Welcome! You are not logged in. [ Login ]
EvC Forum active members: 85 (8937 total)
Current session began: 
Page Loaded: 09-19-2019 3:49 PM
32 online now:
ICANT, PaulK, Tangle, Tanypteryx, Taq, Theodoric (6 members, 26 visitors)
Chatting now:  , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,
Newest Member: ssope
Happy Birthday: AdminPhat
Post Volume:
Total: 861,794 Year: 16,830/19,786 Month: 955/2,598 Week: 201/251 Day: 30/59 Hour: 6/3


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
1
2Next
Author Topic:   Size of the Universe
Hoof Hearted
Junior Member (Idle past 3386 days)
Posts: 24
From: Chorley, Lancs, UK
Joined: 03-20-2007


Message 1 of 22 (493426)
01-08-2009 7:00 PM


I have no relevent qualifications, so this may be a stupid question. Please go easy on me.

Some time ago I posted a question asking how it could be that the observable universe is 156 Billion light years across, when the Universe itself is only 14.5 Billion years old and nothing can travel faster the light. Of course the answer was 'Inflation'. When the 4 forces were unified into a single force, the early universe expanded much faster than the speed of light. Then presumably all the stars in the universe formed long after inflation had ceased.

Ok, so that explains how the universe came to be it's present size. But how is it possible that we can see objects which are so far away? How did light from stars at the edge of the observable universe travel here in order that we could see them? Surely it must be the case that if an object is 75 billion light years away, it must take 75 billion years for their light to reach us?

Edited by Hoof Hearted, : No reason given.


Replies to this message:
 Message 3 by onifre, posted 01-09-2009 2:57 AM Hoof Hearted has not yet responded
 Message 4 by Rrhain, posted 01-09-2009 3:12 AM Hoof Hearted has responded
 Message 8 by cavediver, posted 01-09-2009 6:01 AM Hoof Hearted has responded

    
AdminNosy
Administrator
Posts: 4754
From: Vancouver, BC, Canada
Joined: 11-11-2003


Message 2 of 22 (493433)
01-08-2009 7:49 PM


Thread moved here from the Proposed New Topics forum.
  
onifre
Member (Idle past 1209 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


Message 3 of 22 (493480)
01-09-2009 2:57 AM
Reply to: Message 1 by Hoof Hearted
01-08-2009 7:00 PM


Hi Hoof Hearted,

Im no expert either but,

Some time ago I posted a question asking how it could be that the observable universe is 156 Billion light years across, when the Universe itself is only 14.5 Billion years old and nothing can travel faster the light.

I see a problem with this only in that the circumference of the universe has nothing to do with the amount of 'time' the universe has been expanding.

It is 156 bilion light years across(I'll assume that number is right), and it is also 13.7 billion years old.

But how is it possible that we can see objects which are so far away?

If you mean see as in 'look up', sorry to say but you don't see very far.

You follow by asking,

How did light from stars at the edge of the observable universe travel here in order that we could see them?

Yes, but they don't see that far, they use high powered telescopes to see far distances.

Surely it must be the case that if an object is 75 billion light years away, it must take 75 billion years for their light to reach us?

Yes it is.

I hope that got the thread started, and I hope I was right on most of it.

Oni


"I smoke pot. If this bothers anyone, I suggest you look around at the world in which we live and shut your mouth."--Bill Hicks

"I never knew there was another option other than to question everything"--Noam Chomsky


This message is a reply to:
 Message 1 by Hoof Hearted, posted 01-08-2009 7:00 PM Hoof Hearted has not yet responded

    
Rrhain
Member (Idle past 130 days)
Posts: 6349
From: San Diego, CA, USA
Joined: 05-03-2003


Message 4 of 22 (493481)
01-09-2009 3:12 AM
Reply to: Message 1 by Hoof Hearted
01-08-2009 7:00 PM


Hoof Hearted writes:

quote:
Of course the answer was 'Inflation'.

Incorrect. It isn't "inflation." Rather, it is "expansion." The inflationary period of the universe was a brief moment. However, the universe is still expanding. That means that if you start on one edge of the universe and head toward the other, the other edge is getting further away from you as you travel and thus you will have crossed a longer distance to reach the other side than what was when you first started.

Thus, a photon that started the journey nearly 14 billion years ago to reach us would have had to have crossed 78 billion light years to reach us due to the expansion of the universe. Thus, 78 + 78 = 156 billion light years across.

And getting bigger all the time.

Universe Measured: We're 156 Billion Light-years Wide!


Rrhain

Thank you for your submission to Science. Your paper was reviewed by a jury of seventh graders so that they could look for balance and to allow them to make up their own minds. We are sorry to say that they found your paper "bogus," specifically describing the section on the laboratory work "boring." We regret that we will be unable to publish your work at this time.
This message is a reply to:
 Message 1 by Hoof Hearted, posted 01-08-2009 7:00 PM Hoof Hearted has responded

Replies to this message:
 Message 5 by Hoof Hearted, posted 01-09-2009 3:36 AM Rrhain has responded
 Message 7 by cavediver, posted 01-09-2009 5:58 AM Rrhain has not yet responded

    
Hoof Hearted
Junior Member (Idle past 3386 days)
Posts: 24
From: Chorley, Lancs, UK
Joined: 03-20-2007


Message 5 of 22 (493482)
01-09-2009 3:36 AM
Reply to: Message 4 by Rrhain
01-09-2009 3:12 AM


If a photon started its journey 13.7 billion years ago, how can it travel across 78 billion light years of space in that time?

Yes the Universe is expanding, but it is expanding at sub-light speeds, therefore the majority of the size of the Universe must have been accomplished during the inflation period. My logic tells me that less 13.7 billion light years of expansion would have occured in the 13.7 billion years since the end of the inflation period. So at the end of the inflation period, the universe must have been at least 128 billion light years across. The stars formed after the inflation period. So photons from the edge of the observable universe must have travelled across more than 64 billion light years of space to reach us. How can light travel this distance in less than 13.7 billion years? Obviously there is an error in my logic somewhere.


This message is a reply to:
 Message 4 by Rrhain, posted 01-09-2009 3:12 AM Rrhain has responded

Replies to this message:
 Message 6 by cavediver, posted 01-09-2009 5:04 AM Hoof Hearted has not yet responded
 Message 14 by Rrhain, posted 01-09-2009 4:42 PM Hoof Hearted has not yet responded

    
cavediver
Member (Idle past 1901 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 6 of 22 (493494)
01-09-2009 5:04 AM
Reply to: Message 5 by Hoof Hearted
01-09-2009 3:36 AM


My laptop just bluescreened!! Haven't seen one of those in an age. And I lost my message :mad: So let's try again...

Yes the Universe is expanding, but it is expanding at sub-light speeds

No, expansion is not a velocity so you cannot compare it to the speed of light. Expansion is a measure of how much distance is gained over a set distance per time interval, e.g 10cm per kilometre per second. This has units of T-1 and is clearly not a velocity. If we pick a distance of 1010km, then this distance is expanding at a rate of 109m/s, which is indeed much larger than c, which is 108m/s. BUT, just because they have the same units does not mean they are the same thing!

This is all a consequence of curved space-time. Two object, situated 1010km apart, each consider themselves stationary with respect to their surroundings. Yet the distance between them is increasing at a rate of 109m/s !! However, it would not be particularly sensible to describe them as moving away from each other at 10 x the speed of light. You cannot naively compare the relative velocity of two objects widely separated in a curved space.

This is equally true on the surface of the Earth. Imagine two speed boats on the equator, one at 10oE travelling east at 60knots, and one at 170oW travelling at 60knots west. What is their relative velocity?

Edited by cavediver, : No reason given.


This message is a reply to:
 Message 5 by Hoof Hearted, posted 01-09-2009 3:36 AM Hoof Hearted has not yet responded

  
cavediver
Member (Idle past 1901 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 7 of 22 (493495)
01-09-2009 5:58 AM
Reply to: Message 4 by Rrhain
01-09-2009 3:12 AM


Thus, a photon that started the journey nearly 14 billion years ago to reach us would have had to have crossed 78 billion light years to reach us due to the expansion of the universe.

No, for several reasons. Hoof Hearted's 156 billion is not correct, nor is your article, which doesn't help...

We can only see back as far as recombination (about 400,000 years after the Big bang) as the the Universe is opaque before this time. We see photons today in the Cosmic Microwave Background Radiation that were emitted at recombination, and these define the observable edge of the Universe. This edge is currently about 46 billion light years away. Using observationally consistent values for the expansion model, we can estimate that the photons were emitted at a distance of around 36 *million* light years from us (or where we will be many eons later!)

A photon received *today* from the edge of the Observable Universe, started out 36 million light years from us, and has taken 13.7 billion years to reach us because space has been expanding the whole time. This edge is *now* 46 billion light years away, but that is largely irrelevant to us.

So the photons have travelled either 36 million light years, or 13.7 billion years, but not 46 billion light years, nor 76 billion light years, and most emphatically NOT 156 billion light years :eek:


This message is a reply to:
 Message 4 by Rrhain, posted 01-09-2009 3:12 AM Rrhain has not yet responded

Replies to this message:
 Message 16 by fallacycop, posted 01-10-2009 3:10 AM cavediver has responded

  
cavediver
Member (Idle past 1901 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 8 of 22 (493496)
01-09-2009 6:01 AM
Reply to: Message 1 by Hoof Hearted
01-08-2009 7:00 PM


Surely it must be the case that if an object is 75 billion light years away, it must take 75 billion years for their light to reach us?

In flat space, yes. In a curved expanding space, it will take much much longer, or quite possibly never actually be able to reach us. But as I explained above, the photons started out only 36 million light years away, and have taken 13.7 billion years to reach us because of the expansion slowing their prgress to us.


This message is a reply to:
 Message 1 by Hoof Hearted, posted 01-08-2009 7:00 PM Hoof Hearted has responded

Replies to this message:
 Message 9 by Hoof Hearted, posted 01-09-2009 8:42 AM cavediver has responded

  
Hoof Hearted
Junior Member (Idle past 3386 days)
Posts: 24
From: Chorley, Lancs, UK
Joined: 03-20-2007


Message 9 of 22 (493512)
01-09-2009 8:42 AM
Reply to: Message 8 by cavediver
01-09-2009 6:01 AM


Ok, i'm getting to grips with this I think. However I think it's not quite true to say that it took 13.7 billion years for the light to reach us from the edge of the observable universe. I say this because the first stars did not form until 100 million years after recombination. So the light which reaches us now, began it's journey after these stars formed.
This message is a reply to:
 Message 8 by cavediver, posted 01-09-2009 6:01 AM cavediver has responded

Replies to this message:
 Message 10 by cavediver, posted 01-09-2009 8:54 AM Hoof Hearted has not yet responded
 Message 11 by Larni, posted 01-09-2009 9:01 AM Hoof Hearted has responded

    
cavediver
Member (Idle past 1901 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 10 of 22 (493518)
01-09-2009 8:54 AM
Reply to: Message 9 by Hoof Hearted
01-09-2009 8:42 AM


The photons in the CMBR are not from star-light. They are spontaneous emission from the plasma that was about to recombine. So they are indeed from 13.7 billion years ago.

ABE first starlight would date to 13.6 billion years ago...

Edited by cavediver, : No reason given.


This message is a reply to:
 Message 9 by Hoof Hearted, posted 01-09-2009 8:42 AM Hoof Hearted has not yet responded

  
Larni
Member
Posts: 3990
From: Liverpool
Joined: 09-16-2005
Member Rating: 7.2


Message 11 of 22 (493520)
01-09-2009 9:01 AM
Reply to: Message 9 by Hoof Hearted
01-09-2009 8:42 AM


I'm no expert here, but I think you could be confusing starlight with CBMR.

Curses! Ninja'd!

Edited by Larni, : No reason given.


This message is a reply to:
 Message 9 by Hoof Hearted, posted 01-09-2009 8:42 AM Hoof Hearted has responded

Replies to this message:
 Message 12 by Hoof Hearted, posted 01-09-2009 9:03 AM Larni has responded

    
Hoof Hearted
Junior Member (Idle past 3386 days)
Posts: 24
From: Chorley, Lancs, UK
Joined: 03-20-2007


Message 12 of 22 (493522)
01-09-2009 9:03 AM
Reply to: Message 11 by Larni
01-09-2009 9:01 AM


Yes that is my mistake then. I was under the impression that Hubble had seen stars that were at the edge of the known universe. Obviously very badly informed.
This message is a reply to:
 Message 11 by Larni, posted 01-09-2009 9:01 AM Larni has responded

Replies to this message:
 Message 13 by Larni, posted 01-09-2009 9:08 AM Hoof Hearted has not yet responded

    
Larni
Member
Posts: 3990
From: Liverpool
Joined: 09-16-2005
Member Rating: 7.2


Message 13 of 22 (493523)
01-09-2009 9:08 AM
Reply to: Message 12 by Hoof Hearted
01-09-2009 9:03 AM


No worries, mate. I'm impressed I actually got something right :)

For the record I used to think that time was expanding like space (I know, I know, what was I thinking?).

This is a really good place to learn (even if it does make my head spin, from time to time.)


This message is a reply to:
 Message 12 by Hoof Hearted, posted 01-09-2009 9:03 AM Hoof Hearted has not yet responded

    
Rrhain
Member (Idle past 130 days)
Posts: 6349
From: San Diego, CA, USA
Joined: 05-03-2003


Message 14 of 22 (493595)
01-09-2009 4:42 PM
Reply to: Message 5 by Hoof Hearted
01-09-2009 3:36 AM


Hoof Hearted responds to me:

quote:
If a photon started its journey 13.7 billion years ago, how can it travel across 78 billion light years of space in that time?

I'm sorry. I was a bit unclear. Again, the universe is expanding. Not only is your destination receding from you, your point of origin is, too. Thus, assuming a static universe, the light traveled nearly 14 billion light years. But because the universe expanded while the light was traveling, that point of origin is now 78 billion light years away. The photon covered every point in between, but the points weren't always so close.

quote:
Yes the Universe is expanding, but it is expanding at sub-light speeds

That doesn't matter. It is still expanding. Light is fast, but the universe is huge. It still takes a long time to get across it.

quote:
My logic tells me that less 13.7 billion light years of expansion would have occured in the 13.7 billion years since the end of the inflation period.

Then you need to put that logic aside. The expansion is occurring everywhere at once. You can't think of it in three-dimensional terms. Space doesn't work like that. You're assuming a fixed frame of reference and as we have long since learned from relativity, there is no such thing.

quote:
So at the end of the inflation period, the universe must have been at least 128 billion light years across.

Incorrect. As the article pointed out, a million years after the Big Bang, the universe was 1000 times smaller than it is now, which if the calculations are correct, would put it at 150 million light years across.

quote:
So photons from the edge of the observable universe must have travelled across more than 64 billion light years of space to reach us. How can light travel this distance in less than 13.7 billion years?

Because when they started, the universe wasn't that big. During the journey, the universe expanded. There's a thought experiment that can help you visualize it, but not completely because it, too, assumes a fixed frame of reference:

Assume you're on one end of a rubber sheet, trying to get to the other side. You start walking at a constant rate toward it. As you walk, the sheet stretches, pulling the two ends further and further apart. It is going to take you much longer to reach the other end.

quote:
Obviously there is an error in my logic somewhere.

It's because you are assuming a static frame of reference to measure against. There is no such thing. And with space itself expanding everywhere at once, distance and time cannot be measured linearly.


Rrhain

Thank you for your submission to Science. Your paper was reviewed by a jury of seventh graders so that they could look for balance and to allow them to make up their own minds. We are sorry to say that they found your paper "bogus," specifically describing the section on the laboratory work "boring." We regret that we will be unable to publish your work at this time.
This message is a reply to:
 Message 5 by Hoof Hearted, posted 01-09-2009 3:36 AM Hoof Hearted has not yet responded

Replies to this message:
 Message 15 by DevilsAdvocate, posted 01-09-2009 10:34 PM Rrhain has not yet responded

    
DevilsAdvocate
Member (Idle past 1359 days)
Posts: 1548
Joined: 06-05-2008


Message 15 of 22 (493629)
01-09-2009 10:34 PM
Reply to: Message 14 by Rrhain
01-09-2009 4:42 PM


The seemingly immortal photon
Here is another bizarre thought.

If theoretically you were on the photon proceeding at the speed of light, time would stand still. Meaning that to the light particle it reaches us say 78 billion light years away instantaneously though to us it would take 78 billion light years for it to reach us. That is how relativity works (actually Einstein used flashes of lightning and a moving train as his metaphore for relativity).

This is why some subatomic particles when they are not traveling at the speed of light will decay in fractions of a second but when they are accelerated to near or at the speed of light have an almost infinite lifespan.

Sorry just thought I'd throw that interesting bit of quantum trivia in.


For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
Dr. Carl Sagan
This message is a reply to:
 Message 14 by Rrhain, posted 01-09-2009 4:42 PM Rrhain has not yet responded

  
1
2Next
Newer Topic | Older Topic
Jump to:


Copyright 2001-2018 by EvC Forum, All Rights Reserved

™ Version 4.0 Beta
Innovative software from Qwixotic © 2019