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Space
By BadDoggie (Sun May 18, 2008 at 09:54:26 AM EST) science, universe, astrophysics (all tags)
If the universe is 13.7 billion years old, how can it possibly be 56 billion light-years across? Since the speed of light is constant and cannot have changed, the maximum size of the the universe measured between its two most distant points should be no more than 27.4 billion light-years wide. I'll explain a little further inside.

The speed of light (most likely) hasn't changed since the creation of the universe. A couple videos in cdk007's series, Why Young Earth Creationists Must Deny Gravity explain some of the problems with light speed changes, namely impossible violations of E=mc2 and F=mA. Hell, even "Answers in Genesis" accepts this because of the magnitudes of difference required.

Thanks to Einstein we know that mass is directly related to the speed of light and therefore any increase in the speed of light requires an inverse-squared amount of mass reduction to balance the energy equation (since energy cannot be created nor destroyed). I'm down with that. With less mass gravity then becomes magnitudes of exponentiality weaker and everything falls apart.

So we can be pretty certain that light's speed hasn't changed, not over the long term, not even as a "blip". I don't think anyone can argue with this, certainly not me.

But that leaves us with the problem of a universe which has expanded more than twice the amount it should have been able to. The best attempt at an explanation I've had so far have revolved around violation of the Theory of Relativity: "The light's kind of surfing along with the expansion, so moving at its own speed plus that of the expanding time-space."

BZZT! Fail. If I'm traveling in a spaceship at 0.9999999999c (ignoring the fact that the gravity would crush me to near-singularity), if I switch on a flashlight, relative to an observer the light's moving away from that observer at a speed only 0.000000000001% faster than I am. My spacecraft's speed isn't added to the light. Taking that further, if I could manage to exceed the speed of light by 4ft/sec (walking speed) and I flipped on the headlights, I couldn't notice any change in front of me nor could I even see the trail of light behind me since it'll never catch up to me.

Light's speed in a vacuum is the ultimate speed limit and it can't be changed. Since nothing can move faster than it how can the universe have expanded to its current dimensions?

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Need an astrophysicist here | 24 comments (24 topical, 0 hidden) | Trackback
There's kind of an explanation by TheophileEscargot (2.00 / 0) #1 Sun May 18, 2008 at 10:03:24 AM EST
Here.

You have to remember that space itself is expanding. Think of the photon that's been travelling at the speed of light for 13.7 billion years. Ok, for the first billion years it would have covered 1 billion light-years. But that was 12.7 billion years of expansion ago. So by now, that 1 billion light-years has now expanded so it's a lot bigger.
--
"Life is too short to be interested in everything, but it is good to be interested in as many things as are necessary to fill our days."-Bertrand Russell


Nope. by BadDoggie (1.00 / 1) #2 Sun May 18, 2008 at 10:47:59 AM EST
You're playing the Additive Game which viollates Relativity and the speed limit, basically what I've explained in the details: the light travels at speed c regardless of any other motion. It can't move faster, it can't get a boost, It may red- or blue-shift, but that's frequency, not speed. Light can't hitchhike on the expanding "ether".

woof.

OMG WE'RE FUCKED! -- duxup ?
[ Parent ]

I never said it did by TheophileEscargot (4.00 / 1) #3 Sun May 18, 2008 at 11:05:00 AM EST
Think about the first mile your photon travelled, from point A to point B. Since space has been expanding since then, that mile is now much longer than a mile, say 10 miles.

That doesn't mean that back when the photon traversed it, the photon was travelling at ten times the speed of light. The elastic has stretched since then. If the photon went back, it would now take ten times as long to cover B to A.
--
"Life is too short to be interested in everything, but it is good to be interested in as many things as are necessary to fill our days."-Bertrand Russell
[ Parent ]

Maybe that needs clarifification by TheophileEscargot (4.00 / 3) #5 Sun May 18, 2008 at 11:54:16 AM EST
Suppose the first mile that this photon covered was from A to B. This was 13.7 billion years ago. Say for the sake of argument, A to B used to be 1 mile, but by now has grown to 10 miles.

Now take a mile in the middle of the journey, say from L to M. Back when the photon passed through it, that was just a mile. By now though, it's 5 miles.

Now take the last mile that the photon travelled, Y to Z. The universe hasn't had time to expand since then, so that's still 1 mile.

Now if you think about the average mile that the photon has passed through, that average mile is going to be most like that L to M mile. By now that average mile has grown to 5 miles.

So, the photon originally covered 13.7 billion lightyears in distance. On average, those lightyears have now stretched out to 5 lightyears. So the distance the photon has covered has by now stretched out to 5 * 13.7, or 68.5 billion light-years.

That doesn't mean that the photon ever went went faster than light. Back in the day when it moved between those points, they were much closer together.
--
"Life is too short to be interested in everything, but it is good to be interested in as many things as are necessary to fill our days."-Bertrand Russell
[ Parent ]

One answer by ucblockhead (2.00 / 0) #4 Sun May 18, 2008 at 11:17:39 AM EST
One answer is inflation. The idea is that the universe underwent a massive expansion at far greater than the speed of light before settling down to a state where the current laws of physics apply. No one knows how big the actual universe is.

The size of the Observable universe is explained here.
----
ウセーバラケダ


As a PhD astrophysicist I hereby state by joh3n (4.00 / 2) #6 Sun May 18, 2008 at 12:13:12 PM EST
you should see what ucblockhead said.

Also, you should be careful when you play the adding velocities game.  The only requirement that the universe has is that all observers agree on the speed of light.  It's perfectly allowed for two pieces of the universe to not be in causal contact, just as long as they both agree on the rules.

----
I just ate about 7 pounds of meat
-theantix


*sniff* by ni (4.00 / 2) #7 Sun May 18, 2008 at 12:43:14 PM EST
It's perfectly allowed for two pieces of the universe to not be in causal contact

Is that why theantix no longer posts? Is his universe no long in causal contact with husis?

Is this all just about love, joh3n? Is ALL astrophysics secretly above love?


"These days it seems like sometimes dreams of Italian hyper-gonadism are all a man's got to keep him going." -- CRwM
[ Parent ]

Love by joh3n (2.00 / 0) #10 Sun May 18, 2008 at 01:51:11 PM EST
and pie.

----
I just ate about 7 pounds of meat
-theantix
[ Parent ]

I have to agree about the pie part by R343L (4.00 / 1) #14 Sun May 18, 2008 at 02:30:53 PM EST
But pie is also love. I'm not sure we have this reduced right.

"There will be time, there will be time / To prepare a face to meet the faces that you meet." -- Eliot
[ Parent ]

but what kind of pie? by dev trash (4.00 / 2) #17 Sun May 18, 2008 at 03:08:10 PM EST
Fruit pies are good,  I like blueberry, and raspberry myself.  but a cream pie is also good too.


--
Click
[ Parent ]

it wasn't pie ... by BlueOregon (4.00 / 1) #19 Sun May 18, 2008 at 03:29:55 PM EST

... but I had a creamy rhubarb cake last night (more or a rectangular rhubarb torte, mind you). Mmm ... rhubarb.

_
"The german quoting guy is a little bit out there." (fleece)
[ Parent ]

I have forever been anti-rhubarb by dev trash (2.00 / 0) #23 Mon May 19, 2008 at 06:52:57 PM EST
I am sure the pies and the tortes are good, but as a child we had one kind of rhubarb, boiled.  Ick.

--
Click
[ Parent ]

As a PhD astrophysicist, I hereby state, by ana (4.00 / 2) #8 Sun May 18, 2008 at 12:47:14 PM EST
you should read what joh3n wrote. After all, he's a cosmetoligist, and i'm just a local person gone bad (where "local" means 100 parsecs in this case).

"And this ... is a piece of Synergy." --Kellnerin


As a ex-BSc physicist who's forgotten most of it by TheophileEscargot (4.00 / 2) #9 Sun May 18, 2008 at 01:02:28 PM EST
I think you, joh3n and ucblockhead are wrong when you imply that if it wasn't for inflation, the size of the Universe in light-years would be the age of the universe * 2 as BadDoggie calculates.

The points that the hypothetical photon has passed through, have expanded apart since it passed through them. So you can't just calculate distance = speed * time, since you're adding up a bunch of tiny distances, many of which have grown bigger since the photon passed through them.
--
"Life is too short to be interested in everything, but it is good to be interested in as many things as are necessary to fill our days."-Bertrand Russell
[ Parent ]

Two things by Herring (4.00 / 3) #11 Sun May 18, 2008 at 01:58:47 PM EST
How can you be an ex-BSc? DO they take it away if you don't use it? (Note: although I'm not qualified, I use physics every day for things like walking, seeing etc.)

Also, how can inflationary theory be correct when, in the early universe, there was such tight control of the money supply?

I'm English, and as such I crave disappointment. - Bill Bailey
[ Parent ]

Good point by TheophileEscargot (2.00 / 0) #12 Sun May 18, 2008 at 02:27:35 PM EST
Since nobody's taken my degree away, I suppose technically speaking I'm still a Physicist, despite that fact I hardly remember a thing. It's quite scary really.

Re inflation: you can't spend your way out of a recession because of the laws of economic gravity. However in the early stages of the universe gravity was a trivially weak force.
--
"Life is too short to be interested in everything, but it is good to be interested in as many things as are necessary to fill our days."-Bertrand Russell
[ Parent ]

Er.... by Herring (4.00 / 1) #16 Sun May 18, 2008 at 02:56:53 PM EST
"You can't spend your way out of a recession"

Hang on. A recession is defined as something like 3 consecutive quarters of negative economic growth.

Economic growth becomes positive when more economic activity happens - in other words when people spend more.

Ergo the only way out of a recession is spending.

I'm English, and as such I crave disappointment. - Bill Bailey
[ Parent ]

Well spotted by TheophileEscargot (2.00 / 0) #18 Sun May 18, 2008 at 03:15:38 PM EST
That last paragraph was complete bollocks.
--
"Life is too short to be interested in everything, but it is good to be interested in as many things as are necessary to fill our days."-Bertrand Russell
[ Parent ]

Ah, but that's Hawking Spending by dark nowhere (2.00 / 0) #22 Sun May 18, 2008 at 10:05:35 PM EST
Which requires the spontaneous authorship of IOUs that get sucked in and written off as bad debt at least until the recession is over.

I am not your dupe account.
[ Parent ]

I didn't mean to imply that by ucblockhead (2.00 / 0) #13 Sun May 18, 2008 at 02:29:45 PM EST
I mentioned inflation in that it means that the size of the universe is essentially unrelated to its age and that therefore he was really asking about the observational universe.
----
ウセーバラケダ
[ Parent ]

Sorry, I misunderstood [nt] by TheophileEscargot (2.00 / 0) #15 Sun May 18, 2008 at 02:40:35 PM EST

--
"Life is too short to be interested in everything, but it is good to be interested in as many things as are necessary to fill our days."-Bertrand Russell
[ Parent ]

That's not what I was saying by joh3n (2.00 / 0) #21 Sun May 18, 2008 at 06:31:30 PM EST
Inflation allows for the universe to have the same temperature everywhere, even if the regions are NOW not in causal contact (since they were in contact before inflation, and reached thermal equilibrium then).  Inflation is just an example of how the universes size can be much larger than our observed horizon size.  There are a number of different ways to have the universe be larger than the horizon size.

----
I just ate about 7 pounds of meat
-theantix
[ Parent ]

Um... by NoMoreNicksLeft (2.00 / 0) #20 Sun May 18, 2008 at 05:10:19 PM EST
I think that even time itself has little meaning, when we're talking about velocities like this.

It can't be relative to Earth, because Earth is less than 5 billion. And what other reference point even has meaning?

I don't know if 14 billion is the correct age... it's good enough for me that it's far larger than 6000 years, or whatever the stone age mythology says it is.

Also, you should consider that one of the components of c seems to have changed in just the last few millions years, there is evidence of this being the case. One of the natural nuclear reactors in Africa seems to confirm this, as does light passing through some nebula whose name escapes me at the moment.

It's a strange universe.
--
Do not look directly into laser with remaining good eye.


Magic and little pixies by DullTrev (2.00 / 0) #24 Mon May 19, 2008 at 09:57:08 PM EST

And, as has been said elsewhere, inflation. The speed of light is the ultimate speed limit for travelling through space-time. Space-time itself can expand at whatever damn speed it feels like, apparently.

Inflation is a great theory, because it makes other problems go away, leaving you only with the problem of how the feck inflation could have happened. This is why you should love cosmology.

Sometimes I wish I'd actually done some work and been a proper scientist, instead of getting all my information from The Sky At Night.


--
DFJ?


Need an astrophysicist here | 24 comments (24 topical, 0 hidden) | Trackback

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