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Does Space Expand?

by Wallace
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Drakkith
#163
Mar27-12, 12:52 PM
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Quote Quote by lostprophets View Post
i respect your guess.
if light was more abundant than darkness at the start where as the reverse is true now, am i to believe then that the universe is getting smaller?
No, as has been repeatedly said, the universe is expanding.

what if it was expanding and contracting
There's no need to ask "what ifs" that aren't real. The universe is expanding, not contracting.

i ask about light "pushing" darkness (light pressure) clearing a path .
I don't know what you don't understand about darkness simply being the absence of light. Darkness is an abstract concept linked to vision. If a volume of space is completely devoid of EM radiation (light) we do not call it dark, we call it empty of radiation. Light propagates through space and interacts with matter. It cannot interact with empty space as there is nothing to interact with!
so if we had darkness first with energy
We did not have darkness first. As I said light has existed since the earliest moments of the universe when the density and temperature of the universe was so high that matter and antimatter was continually being created from EM radiation and annihilated, converting back to EM radiation.

then light energy appears,room has to be made for this light.
It did not "appear". The energy already existed. Furthermore you keep suggesting that "darkness" is something physical and tangible. It is not. Does a vacuum have to make room for particles to exist in it? No!

could light then clear this "room" creating a vacuum redundant of energy once this light has lost its energy and gone..this then takes time to rebuild itself with dark energy matter, un til it over crowds sparking another light source and repeats the process.
Absolutely not. The earliest moments of the universe was full of interacting particles and radiation. As George said above, imagine being inside the core of the Sun, but a billion billion trillion times denser and hotter. Then go another quadrillion above that. Then you will be getting close to the state of the early universe.

i could be way off and have no idea what im on about.but ive read some say that the universe is expanding fast than light... how do we measure this.do we measure it with light?
if light is "pushing" then light will always be behind therefore it could be seen that anything infront of it is moving fast when really its not
We measure it by looking at the amount of redshift an object presents to us. The further away an object such as a galaxy is, the more its light is redshifted. This is due to the expansion of the universe causing it to recede from us and stretching out the light as it travels over billions of years.

Also, the expansion of the universe is a "rate", not a measurement of velocity. What this means is that objects further away will accelerate away from us quicker than objects closer to us will. The speed at which objects move away from us is called the recession velocity. Currently our measurements show that this recession velocity increases by about 70 km/s per megaparsec (3.26 million light-years) in distance that an object is from us. So a galaxy at 2 megaparsecs in distance from us would be receding at about 140 km/s, while a galaxy at 20 megaparsecs would recede at 1400 km/s. If the rate of expansion were higher, the recession velocity would increase by a larger amount per distance, such as being 100 km/s per megaparsec.
lostprophets
#164
Mar27-12, 12:53 PM
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sorry . i did mean to use the word "front" lightly .ooosh pun ,not so poetic...
yes theres no front, back, middle, only edges,curves,surrounding, enclosed ,in a tomb of darkness..
the further we look back the brighter it gets. its logic for it to be so. but is it logic to think that what one is looking at is not the beginning but a random?

thanks for the reply ...

also how far can my eyes see. meaning
when i see light that has come from a far distance.at what distance am i seeing it.?
am i seeing the light from the distance of my eye or am i seeing the light light years away. my eyes can see distances.so i ask is it possible to travel down the light to the source and bring it nearer?
i no i may like a fruit loop hope you dont mind..
my question is this.
is the light seeing me or am i seeing it?

also i went to the optitions today.he put a light in my eye .when this light was taken away i had a dark line of vision.i asked why.
he said its because the light removes something or other ,sorry cant remember exactly,so there was an empty space .but
over time the energy recovers and bring the light back to this dark spot...can space work the same?
meaning does light remove matter then once the light has gone this matter returns over a time period...
Drakkith
#165
Mar27-12, 01:09 PM
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Quote Quote by lostprophets View Post
the further we look back the brighter it gets. its logic for it to be so. but is it logic to think that what one is looking at is not the beginning but a random?
Well, your first sentence is incorrect. It is not brighter the further we look back. After the universe formed it cooled and expanded over millions of years. Finally after the temperature and density dropped beyond a critical point, protons could combine with electrons, forming neutral atoms that are mostly transparent to light. Before this point in time light could not travel more than mere nanometers before interacting with protons or electrons. After this point in time the universe became "transparent", and the radiation that was released from electrons combining with protons could suddenly travel over light-years and is currently seen as the Cosmic Microwave Background. The CMB is literally the furthest back we can see using light. It is not physically possible to see beyond this point unless we can somehow invent a neutrino detector in the future that is a few trillion times more sensitive than current ones.

After this recombination, atoms could finally start to collapse under gravitational attraction to form the first stars and galaxies. Whether the universe is "brighter" now or then is unknown to me.

I don't know what your asking with the 3rd sentence.

also how far can my eyes see. meaning
when i see light that has come from a far distance.at what distance am i seeing it.?
am i seeing the light from the distance of my eye or am i seeing the light light years away. my eyes can see distances.so i ask is it possible to travel down the light to the source and bring it nearer?
i no i may like a fruit loop hope you dont mind..
my question is this.
is the light seeing me or am i seeing it?
I don't really know what you are asking. Since photons can travel through space for billions of years, if your eye detects one then you are seeing something billions of lightyears away. The only limit to how far an object can be seen is simply that the universe is only a finite age. The CMB was released over 13 billion years ago, so as time passes the area of space that those photons we see were released from is getting further away.
lostprophets
#166
Mar27-12, 01:50 PM
P: 29
im now confused. one of you is saying the further you go back the brighter it gets,and the other is saying not so...
the problem may stem from the "no matter which direction we look ,it all looks the same" on a large scale not small...
so how do we get around this.?
its like looking at a field full of sheep and guessing which one came first.
neutrinos collide with things at random points at random times......this to me is very important.
Drakkith
#167
Mar27-12, 02:09 PM
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Quote Quote by lostprophets View Post
im now confused. one of you is saying the further you go back the brighter it gets,and the other is saying not so...
Sorry. Before the CMB was emitted you did in fact get "brighter" the further back you go, but that is kind of inaccurate as the state of the universe was very different from what it is today. I prefer the terms "hotter" and "denser".

the problem may stem from the "no matter which direction we look ,it all looks the same" on a large scale not small...
so how do we get around this.?
its like looking at a field full of sheep and guessing which one came first.
neutrinos collide with things at random points at random times......this to me is very important.
Get around what? The universe is very homogenous on the large scale.

And may I request that you make specific questions. Much of your posts seem to be ramblings that don't make any sense and don't seem to be asking anything. It would help both us and yourself if you could trim your posts down to clear, concise questions.
Drakkith
#168
Mar27-12, 02:13 PM
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Quote Quote by lostprophets View Post
also i went to the optitions today.he put a light in my eye .when this light was taken away i had a dark line of vision.i asked why.
he said its because the light removes something or other ,sorry cant remember exactly,so there was an empty space .but
over time the energy recovers and bring the light back to this dark spot...can space work the same?
meaning does light remove matter then once the light has gone this matter returns over a time period...
The bright light uses up the chemicals in your eye that respond to light, allowing you to see. These chemicals require time to be replaced in your cells, so it takes a little bit for your vision to return to normal. The light isn't pushing anything out of the way and the chemicals are still there, they are just used up in a reaction that turns them into something else.
lostprophets
#169
Mar27-12, 02:53 PM
P: 29
Quote Quote by Drakkith View Post
The bright light uses up the chemicals in your eye that respond to light, allowing you to see. These chemicals require time to be replaced in your cells, so it takes a little bit for your vision to return to normal. The light isn't pushing anything out of the way and the chemicals are still there, they are just used up in a reaction that turns them into something else.
yes..
i see. thank you. the light turns them into something else.....1+1 = 3
GeorgeDishman
#170
Mar27-12, 04:49 PM
P: 255
Quote Quote by lostprophets View Post
im now confused. one of you is saying the further you go back the brighter it gets,and the other is saying not so...
the problem may stem from the "no matter which direction we look ,it all looks the same" on a large scale not small...
so how do we get around this.?
its like looking at a field full of sheep and guessing which one came first.
neutrinos collide with things at random points at random times......this to me is very important.
It is summer and you are in a field of sheep, all born in the same week in the spring. If light travels slowly, you see old sheep near you but lambs far off. When we look far away, we see the universe as it was earlier. It was brighter earlier.
Drakkith
#171
Mar27-12, 05:16 PM
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Quote Quote by lostprophets View Post
yes..
i see. thank you. the light turns them into something else.....1+1 = 3
What's up with your equation? Is something confusing or are you joking?

Quote Quote by GeorgeDishman View Post
It is summer and you are in a field of sheep, all born in the same week in the spring. If light travels slowly, you see old sheep near you but lambs far off. When we look far away, we see the universe as it was earlier. It was brighter earlier.
I am uncertain if you can say that the universe was brighter earlier. I think that depends on the amount of light being output by stars at various times in the universe. If there are more stars now than in the past, today may be "brighter". Either way it's a confusing issue that doesn't really say much.
GeorgeDishman
#172
Mar28-12, 11:56 AM
P: 255
Some context might help:

OP:
so if we had darkness first with energy, then light energy appears,
Me:
For the first 378,000 years, the whole universe looked like the interior of the Sun, the farther back in time you go, the brighter it was.
Quote Quote by Drakkith View Post
I am uncertain if you can say that the universe was brighter earlier. I think that depends on the amount of light being output by stars at various times in the universe. If there are more stars now than in the past, today may be "brighter". Either way it's a confusing issue that doesn't really say much.
A small black body at a "typical" location in the universe now (i.e. probably in intergalactic space) would be at equilibrium somewhere near 3K, at the time of last scattering what we see as the CMBR was at 2975K and our test body would also have had that temperature. If that applied today, the Earth could be no cooler. Before 48,000 years, the universe was the "radiation dominated" era so I think my statement is valid.

If the OP is curious what the sky would have looked like back then, he can put 2975 into the box at the bottom right of this applet:

http://webphysics.davidson.edu/alumn...ava/bb_mjl.htm

The colour is the circle marked "composite" in the left panel.
Drakkith
#173
Mar28-12, 04:13 PM
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Quote Quote by GeorgeDishman View Post
A small black body at a "typical" location in the universe now (i.e. probably in intergalactic space) would be at equilibrium somewhere near 3K, at the time of last scattering what we see as the CMBR was at 2975K and our test body would also have had that temperature. If that applied today, the Earth could be no cooler. Before 48,000 years, the universe was the "radiation dominated" era so I think my statement is valid.
While all true, what I mean is that saying the universe is brighter in the past may only be correct when you look back to a certain point in time. My question is if there are more stars now than in the past, and if those stars are outputting more total light than they were in the past. That combined with the density of the universe could mean it is definately brighter in the past, or not. I have no idea if it was brighter, nor do I know how to find out.
GeorgeDishman
#174
Mar28-12, 05:17 PM
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Quote Quote by Drakkith View Post
While all true, what I mean is that saying the universe is brighter in the past may only be correct when you look back to a certain point in time. My question is if there are more stars now than in the past, and if those stars are outputting more total light than they were in the past. That combined with the density of the universe could mean it is definately brighter in the past, or not. I have no idea if it was brighter, nor do I know how to find out.
The OP was talking about the first instants of the universe and prior to the release of the CMBR, it was brighter earlier. After that is more complex ;-)

Once the plasma combined into neutral hydrogen, there was the period called the "dark ages" because there were no stars at all. Then the first Pop III stars formed perhaps after around 100 million years. They were probably very large and short lived and put a lot of "metals" (elements beyond helium) into the mix when they ended as supernovae. As the proportion of heavy elements increased, stars could be smaller and new star production peaked then fell. All that time the universe was expanding so the density was falling too. Overall, there would have been a peak in stellar brightness when the universe was perhaps 1 to 3 billion years old. The rate of new star production now is conventionally thought to be perhaps one tenth of the peak but that is a point of current debate.
Drakkith
#175
Mar28-12, 05:24 PM
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Hmmm. I have a few ideas/questions but I'll save that for another thread.
Trenton
#176
Mar28-12, 10:47 PM
P: 94
The idea of expanding space seems to me to be precicely as strange as in-falling space in the vacinity of black holes. One is simply the reverse of the other!

Since the big bang appears to be a singularity that for some reason became disrupted and exploded outwards, it would follow that space would expand in a collapsing or diminishing gravitational field. The further out the space is, the faster it expands - Just as with a black hole the further in the faster it shrinks.

This seems to me to be a better explanation than dark energy for the accelerating expansion - not least because we don't have to look for anything new!

And yes it can go faster than light because the speed of space itself is not limited in GR.
Orion1
#177
Mar29-12, 08:55 AM
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Equation of State:
[tex]P(r) = -\rho(r) c^2[/tex]

Inflationary metric:
[tex]ds^2 = - (1 - \Lambda r^2) c^2 dt^2 + \frac{1}{1 - \Lambda r^2} dr^2 + r^2 d\Omega^2[/tex]

If space is still expanding, then would this metric also still be applicable in the Universe at present time?

Reference:
Cosmic inflation - space expands - Wikipedia


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