Universe "at least 156bn light years wide"

[Dave]

Quoted from BBC News:

 

"The Universe is at least 156 billion light-years wide, say astronomers.

 

The estimate comes from data obtained by a space probe that is examining the so-called Cosmic Background Radiation - often called the echo of the Big Bang.

 

The echo contains information of what the cosmos was like when it was young and how it might develop.

 

The cosmos is 13.7bn years old but the stretching of space with its expansion after the Big Bang means that simple distance measurements do not apply."

 

For more information, look at:

 

http://news.bbc.co.uk/1/hi/sci/tech/3753115.stm

[Martin]

Quoted from BBC News:

 

 

For more information' date=' look at:

 

http://news.bbc.co.uk/1/hi/sci/tech/3753115.stm[/quote']

 

I looked at the BBC article and I couldnt find anything radically new.

This writer says particle horizon = 78 billion LY and a couple of years ago cosmologists were saying numbers like around 50. (cant remember exactly)

 

Here is a quote from the BBC article:

"What it means is that the starting point of a particle of light, a photon, reaching us today after travelling for 13.7 billion years is now 78 billion light-years away."

 

I will try to see where the difference comes from, but a small variation in the parameters of the model could explain the difference between around 50 and around 78.

 

Let's try a very large redshift like z = 100,000

in Siobahn Morgan's cosmology calculator.

with the usual parameters like Lambda (dark energy) = 0.73

and Omega (matter) = 0.27

and H = 71

 

I will get the link for Siobahn's calculator, it is in the Astronomy forum sticky.

[Martin]

Well, I went to the online calculator

http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html

and put in z = 10,000 and got a distance of 46 billion LY.

So my first reaction was wrong. this number 78 is kind of revolutionarily big.

 

With Morgan's calculator you put in 0.73 for lambda and 0.27 for omega and 71 for H (which are pretty widely accepted values for dark energy, matter(ordinary and dark) and hubble parameter. Then you say z = 10,000 and it tells you the present distance of an object whose light you observe today with a redshift of 10,000.

It says the light has been traveling for 13.7 billion years and it shows the distance to be 46 billion LY (because of space stretching out during travel)

 

Something has changed, for them to be saying 78 instead of 46 (or anyway something around 50 since there is a fair bit of uncertainty it should be plus or minus some confidence interval but lets not worry about that)

 

So I am a little troubled by the 78. Maybe someone will offer some new information.

[Martin]

Whoah! the BBC journalist mistook it. I looked at Niel cornish original paper in Phys Rev. Lett. 21 May 2004 which did NOT say that particle horizon was 78 billion LY

it was talking about the smallest possible size the U could be if it was finite.

 

its either infinite in extent, and flat, or flat and toroidal or it has a slight positive curvature and is finite, but then, because the curvature is so near zero it has to be very big.

 

so cornish and spergel etc estimated how big and they said there could be an antipodalpoint (which we would not be observing now because the light wouldnt have reached us) which is 24 Gpc

a parsec is 3.0857 LY so multiply 24 by that and get 70-some

so that is OK, close enough to 78

 

so its infinite flat

(toroidal doesnt seem likely)

or else if its finite S³ and slightly positive curved then

it cant be any smaller than this Cornish/Spergel distance of

78 billion LY

 

well this took me completely by surprise.

and had me fooled for a while, I am used to the flat infinite model with

a particle horizon (the present distance of something whose light

we imagine getting after 13.7 billion years, rather idealized) of

say 46 billion LY or around 50

 

should start a thread about the Cornish article

[5614]

i find all of this space stuff very interesting, however recently i was learning about some space theories, did you know that it is quite possible that in a few thousand, million or maybe billion years the entire universe could implode on itself.

becuase at the moment the universe is expanding, however if there is enought mass in the universe know as the critical mass by scientists [unknown value] then using gravity all of the solar systems and galaxies will all start attracting each other and in the end the whole universe will end up how it was. if this theory is correct, then it is quite possible that this process has been happening for billions of years.

alternatively, if the mass of the universe is less than the critical mass, then the universe will expand forever.

however we [humans] are unlikely ever to find out the reality, as our sun will die before any of this happens... presumably that is!

can anyone explain in a lot of detail about black matter or dark matter? the stuff which cant be detected but mathematically has to be there! ive read some basic stuff about it, but want more details please... thanks

[Qwik]

Somebody correct me if I'm wrong, I don't know a whole lot on this subject, but..

 

Omega, the ratio of the observed density of the universe to the criticle density of the universe (the density required to stop the expansion), is the deciding factor in the fate of the universe. If omega is less than 1, the universe will keep expanding. If it's greater than 1, the universe will slow down, and start to contract, the "ending" would be the "Big Crunch." If omega is equal to one, the universe will stop expanding, but will not start to contract.

 

The reason I have "ending" in quotes is because some believe that after the "Big Crunch" another "Big Bang" will occur, and the cycle will be started all over again.

[Martin]

i find all of this space stuff very interesting' date=' however recently i was learning about some space theories, did you know that it is quite possible that in a few thousand, million or maybe billion years the entire universe could implode on itself.

becuase at the moment the universe is expanding, however if there is enought mass in the universe know as the critical mass by scientists [unknown value'] then using gravity all of the solar systems and galaxies will all start attracting each other and in the end the whole universe will end up how it was. if this theory is correct, then it is quite possible that this process has been happening for billions of years.

alternatively, if the mass of the universe is less than the critical mass, then the universe will expand forever.

however we [humans] are unlikely ever to find out the reality, as our sun will die before any of this happens... presumably that is!

can anyone explain in a lot of detail about black matter or dark matter? the stuff which cant be detected but mathematically has to be there! ive read some basic stuff about it, but want more details please... thanks

 

 

 

Somebody correct me if I'm wrong' date=' I don't know a whole lot on this subject, but..

 

Omega, the ratio of the observed density of the universe to the criticle density of the universe (the density required to stop the expansion), is the deciding factor in the fate of the universe. If omega is less than 1, the universe will keep expanding. If it's greater than 1, the universe will slow down, and start to contract, the "ending" would be the "Big Crunch." If omega is equal to one, the universe will stop expanding, but will not start to contract.

 

The reason I have "ending" in quotes is because some believe that after the "Big Crunch" another "Big Bang" will occur, and the cycle will be started all over again.[/quote']

 

if no one else wants to respond to these comments i will give it a shot

 

both are essentially right, except 5614 does not mean "critical mass"

but "critical density"

which is so-and-so much mass (or energy) per cubic kilometer

 

it is simplest to imagine the U as infinite so it would not have a finite amount of mass, but it would have a density that you could measure or at least estimate

 

and the critical density has been estimated at 0.83 joules per cubic km

or the equivalent in whatever other units you like (5614 says it is not known but cosmologists have a pretty good handle on it)

 

and the critical density is the density required for spatial flatness

and recent WMAP observation has made people confident that the U is

either exactly or pretty nearly flat

so that the actual density is within a percent or so of critical

 

cosmology has made bigtime progress in the past 5 or 6 years, since Wendy Freedman's team measured the hubble parameter in 1998 and the supernova data came in also that year---it has made incredible progress.

 

so one can actually say that the hubble parameter is 71 (plus/minus a modest uncertainty) km/s per Mpc

and that 71 translates directly into the 0.83 joules per km figure for the density. so one can actually say in ordinary everyday language what the density of the U is.

 

well anyway it impresses me

 

and one consequence of this growing certainty about the main parameters is that the U does not have enough density to cause itself to collapse

 

so what 5614 and QWIK mention about collapsing is not being forecast

by cosmologists

(but one should remember that human theories are merely models and

can be wrong and we dont control nature she controls us---all one can say is that the currently prevailing model among cosmologists does not predict collapse)

[hagbard celine]

Now if only someone would make a tape measure long enough to test this.