# Age of the Universe

1. Jun 15, 2007

### bassplayer142

In another topic the oldest black hole has been found.

"Sitting at the heart of a distant galaxy, the black hole appears to be about 12.7 billion years old, which means it formed just one billion years after the universe began and is one of the oldest supermassive black holes ever known."

I am wondering how astronomers see and claimed that this black hole formed a billion years after the universe. Say that the universe has a center where it all began. At the big bang everything started accelerating in a spherical shape outwards. If we are somewhere on the side of the sphere then how could something that happened 12.7 billion years ago be seen? And since we are not going faster then the speed of light, the light from the black hole should already be past us. It is kind of hard to explain and if you have any questions just ask.

2. Jun 15, 2007

### Chronos

Oh, but we are going faster than the speed of light wrt to this 'oldest' black hole, just not locally.

3. Jun 15, 2007

### damgo

You're thinking of the big bang as an explosion from a center point, but this is entirely inaccurate. There is no apparent 'center' to the universe: as far as anyone can tell, the universe is the same everywhere and in all directions (homogenous & isotropic).

I do not know how that estimate for this black hole's age was found. If it is located in a very distant galaxy, then it has taken the light from that galaxy quite a long time to reach us, and so we are seeing the galaxy as it was a very long time ago (in a sense.) We generally can't see more than a few billion years "into the past" this way, though -- things further away than that are usually too dim to really make out -- so they must have used some other method to determine its age.

4. Jun 15, 2007

### marcus

Bassplayer, when you give a quote like that it is often helpful, or just plain interesting, if you give a LINK.

In this case I believe I recall seeing this news. It was the current highest-redshift quasar observed. Only slightly higher redshift, slightly further back in time, than the previous champion.

there is no problem. in standard cosmology distances can expand much faster than the speed of light (this is not breaking the speed limit the way a spaceship would if it whizzed past the earth at faster than light)

having distances expand FTL is just something you need to get the model to fit the observational data, no big deal and no violation of special rel.

so yeah, the light from that particular quasar has been traveling towards us for 12.7 billion and it is only just now getting here

we get light from other stuff that has been traveling towards us even longer, like well over 13 billion. For instance the cosmic background radiation light, which is so stretched out by now that it is microwaves.

5. Jun 15, 2007

### marcus

I found news links that could be what BassP is talking about

http://space.newscientist.com/article/dn12007-new-quasar-is-the-oldest-yet.html

http://news.yahoo.com/s/space/20070607/sc_space/mostdistantblackholediscovered [Broken]

according to estimates, quasar is from back when universe was less than 1 billion years old
estimated mass = 1/2 billion solar

"[Willot] told the Canadian Astronomical Society meeting in Kingston, Ontario, that the survey had found four quasars beyond a red-shift of 6, including one, given the memorable title CFHQS J2329-0301, at a record red-shift of 6.43. That means we're seeing it just 870 million years after the big bang."

difficult to explain how object that big could have condensed from gas that early

just as an exercise, I put z = 6.43 into Ned Wright's cosmo calculator and got the following:

* It is now 13.665 Gyr since the Big Bang.
* The age at redshift z was 0.869 Gyr.
* The light travel time was 12.797 Gyr.
* The comoving radial distance, which goes into Hubble's law, is 8601.1 Mpc or 28.053 Gly.
* The comoving volume within redshift z is 2665.310 Gpc3.
* The angular size distance DA is 1157.6 Mpc or 3.7757 Gly.
* This gives a scale of 5.612 kpc/".

the light has been traveling 12.8 billion years
and comes from back when the universe was 0.87 billion years old
at which time the quasar was only 3.8 billion LY away from us (or the crud that eventually made us and the rest of Milky)

and now the quasar, or whatever remains of it (maybe just a big quiet black hole), is 28.1 billion LY away from us

plugging the same redshift into Morgan's calculator gives approximately the same numbers and also give speeds:
the recession speed THEN when the light we are seeing was emitted was 2.89 c
and the recession speed NOW at the present moment (of what remains of the quasar) is 2.03 c

this illustrates the superluminal stretch speeds common in standardard cosmology

the thing they are discussing was receding from us almost at 3 times lightspeed when it emitted the light that arrived here yesterday

Last edited by a moderator: May 2, 2017
6. Jun 15, 2007

### BoomBoom

In an accelerating expansion universe, shouldn't the recession speed (NOW) be more than (THEN)? Or is this due to us being part of a cluster and therefore slowing down our recession?

7. Jun 15, 2007

### damgo

^^^ Yes, that's correct about the recession speed. But the universe only began accelerating relatively recently; before that it was decelerating. (Sort of like a 'bounce.') The 'dark energy' causing the expansion could have been there all along, but the mutual gravitational attraction of matter in the universe was stronger than it until quite recently.

8. Jun 15, 2007

### marcus

good explanation. thanks for clarifying, damgo

here are links to cosmology calculators

Ned Wright's most basic cosmology calculator
http://www.astro.ucla.edu/~wright/CosmoCalc.html

Morgan's calculator
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

maybe just for fun we should look at recession speeds for a smaller redshift case, since then it is more recent, acceleration should have kicked in and we should see an increase in speed instead of a decrease

Yes! I put z = 0.2 into Morgan's and it said the recession speed was 0.24c when emitted and is now 0.28c as the light gets to us.

Part of that increase from 0.24 to 0.28 is simply that the thing is farther away now: quite apart from acceleration, farther things recede faster. Again that is partly compensated by the decline in the Hubble parameter, which is still decreasing even though the growth in scalefactor in accelerating. But some part of the effect is doubtless due to acceleration.

when you stay down around z = 0.2 then you are sure to be in recent times (when dark energy dominates over matter and acceleration takes over) because z = 0.2 corresponds to universe age of 10.3 billion years, only 3.4 some billion years ago.
the past 3-4 billion years has definitely been in the dark energy dominated era---so it is more clear-cut.

if you use Morgan be sure you remember to put the parameters (.27, .73, 71) in. Wright's has them as default so you don't have to remember to do this.

If you put the correct LCDM-model parameters (.27, .73, 71) in you will get out numbers which fit the observations well. (LCDM fits huge masses of data remarkably well and the parameters are determined by data-fitting). If you put in very different parameters you will get unreasonable results, as you might expect.

Last edited: Jun 16, 2007
9. Jun 15, 2007

### Chronos

You can get some bizarre numbers if you mess with the parameters in the calculator. They may not be provably wrong, but, appear highly suspect when you take other observations into account.

10. Jun 16, 2007

### bassplayer142

I still don't get how the universe doesn't have a center where it all began. At the big bang everything was defined at one point right?

11. Jun 16, 2007

### Wallace

No, but this is a very common misconception. The 'Big Bang' is a pretty poor name for the current theory of cosmology, but that's what we a stuck with.

We describe the 'size' of the Universe by the scale factor a(t). If after some period of time say that a doubles, this means the distance between things in the expanding universe have doubled. If you wind the clock backwards then you find that everything was closer together in the past. If we go to t=0 then a=0 and the distance between everything becomes zero.

So then one might ask 'where is the point that everything is sitting at, since the distance between everything is zero'. This is however a mistake, since the Big Bang does not describe the expansion of a bunch of material into previously empty space. That is the most important thing to get your head around. Do not think of the Big Bang as explosion of material happening in a pre-existing space. As the Universe expands there is more 'space' created and hence a t=0 there is no space at all!

Therefore the Big Bang happens everywhere and all points in the Universe look like 'the centre' for observers at that point.

One common way to think about it is this. Imagine that the space in the Universe is represented as the surface of a balloon. If dots are drawn on the balloon (representing say galaxies) then as the balloon is blown up the dots (galaxies) all get further apart, and from each dot every other dot is getting further away. Each point 'looks' like the centre but there is no actual centre. Now consider shrinking the balloon down. We could imaging shrinking it till the surface area of the balloon goes to zero. This is the Big Bang, and there is no centre and the distance between all the dots goes to zero.

12. Jun 16, 2007

### marcus

I agree with Wallace that the (all space = balloon surface) analogy is helpful and will add one comment:

as we usually picture an inflating balloon it HAS a center-----it is surrounded by a space of higher dimension and the R=0 point is in that higher dimension space, not on the balloon surface

but that is "cheating". in the analogy, the balloon surface is our analog of ALL SPACE
so the center of expansion is nowhere in all of space. if you lived in the balloon surface, you could not point at the center of expansion. because all directions your finger can point are in your space.

so there is no center of expansion that you can point towards, anywhere in all of space.

that is one thing that the balloon analogy illustrates

13. Jun 16, 2007

### marcus

Ideas of the big bang are changing, among the experts. The popular media picture has not changed yet, AFAIK.

If you look in a top peer-review journal Physical Review D you see a lot of articles by Martin Bojowald and his collaborators, and also Abhay Ashtekar and his collarborators. In their picture the big bang was preceded by a contracting phase and at high density gravity has quantum corrections making it repelling instead of attracting----so under some conditions you get a bounce.

conditions get pretty extreme during the bounce-----planck-scale pressure, temperature, density etc.-----but the whole universe does not actually go down to a mathematical point.

there was a 3-week top-level workshop about this and related things at Santa Barbara in January this year. most of the talks are online video so you can check them out if you want----but the content is nearly all in technical language.
http://www.kitp.ucsb.edu/activities/auto2/?id=332
click on "talks" for the online videos

A more popular-written essay will be coming out soon in the magazine called Nature Physics. I will watch for it and put a link if I see it. It is not in June issue, but may be in July issue.
http://www.nature.com/nphys/index.html
It may be pay-per-view though. For now I'd advise not to put too much credence in mass media versions of bigbang that have appeared so far.

Last edited: Jun 16, 2007
14. Jun 16, 2007

### Wallace

Quantum gravity, 'bounce' Universes, multiverse and cyclic Universe theories don't change the notion that there is no centre or single initial spatial point that the big bang occurred at. I don't think you meant to say that they do Marcus, but the juxtaposition of the quote you used and your statement that 'ideas are changing' seemed to imply that.

15. Jun 16, 2007

### marcus

that is right Wallace,
the questioner, bassplayer, has TWO areas of confusion and I am trying to address the other one now.

we agree that there is IN SPACE TODAY no specific initial location from which the world expands.
you can't point your finger at a definite somewhere the expansion comes from.

I think we dealt with that---in particular you dealt with it, using the balloon analogy.

But bassplayer still may be hung up on the thought that IN THE PAST there was a point of infinite density from which it all expanded.

I want to assure bassplayer that there is no scientific reason to believe that such a point of infinite density ever existed. The reason is that there are equally good bounce models which reproduce the observations as well as the classical models do---they converge to classical picture quickly after the bounce episode. And bounce models do not involve such a point.

In the bounce models, infinite density is never reached. The highest density ever gets (in Ashtekar's numerical work) is about 80 percent of Planck density, which is high, but not infinite.

Last edited: Jun 16, 2007
16. Jun 16, 2007

### damgo

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

As other posters have said, the Big Bang is an expansion of space, not in space. Think of it this way: distances (between galaxies) are getting bigger everywhere. Things in the universe are all growing further & further apart, but there's apparent no edge or center: the universe looks basically the same everywhere as far as we can see. This picture (just look at the black dots, which would represent galaxies) gives you a good idea of what we mean:

http://www.astro.ucla.edu/~wright/cphotons.gif .

To make this a more mathematical explanation, imagine a one-dimensional toy universe with galaxies located at the points:
$$x_n = n t , \qquad n \mbox{ an integer}$$
where t is the time. So at t=1, there are galaxies at x=0, 1, 2, 3, ... and x=-1, -2, ... . By t=2, these galaxies have moved to x=0, 2, 4, 6, ... and x=-2, -4, .... This is an infinite universe, with an infinite number of galaxies in it. And it's expanding. The distance between nearby galaxies is clearly just t.

In this toy universe, let's "look back" at what happens in this universe at very early times, back near t=0. At earlier & earlier times -- t=0.1, t=0.01, t=0.001, and so on -- the galaxies in the universe become closer & closer together. As we approach t=0, the density approaches infinity. But at this t=0 things look very weird. For any positive time, no matter how small, there are an infinite number of galaxies and space extends infinitely in either direction; but at t=0, all the galaxies are at position $$x_n = 0$$. Clearly t=0 is the "Big Bang' in our toy universe.

---

Two things that bear mentioning: first, it's not clear how seriously we should take the point at t=0. It's not an 'ordinary' point like any other. You can think of it sort of as like the point at the pointy tip of a cone. Imagine the universe (space and time here!) as a cone laying sideways. The distance along the cone from the pointy end represents time. So at any fixed time (not 0), we take a crosswise slice of the cone there and see this universe looks like a circle (btw, it's also closed so if you go far enough in one direction you come back around.) But if we look at a crosswise slice of the cone at t=0 -- at the pointy end itself -- we see that at t=0, the universe doesn't look like a circle, but like a point! We've gone from a one-dimensional universe at positive t, to zero-dimensional universe at the Big Bang. This is just like what happens in the real universe. At t=0, all the galaxies are "at one point", but it doesn't make sense to ask "where" that point is in the universe... that would be just like asking "where along the circle" the pointy tip of the cone is located.

A second thing is that there's no reason at all to trust our current laws of physics to work all the way back to the Big Bang. In fact we pretty much know they won't work at very early times. To truly figure out what happened before ~$$10^{-44}$$ seconds, we need a theory of quantum gravity; extrapolating back with GR just won't cut it. So we don't have any idea what happened in that "first" tiny fraction of a second.. whether there was a true Big Bang, or a bounce like marcus mentioned, or something else entirely.

---

Anyways, do read that SciAm article -- it is very helpful. The pages from Ned Wright's cosmology FAQ below might help you, too.

http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

http://www.astro.ucla.edu/~wright/nocenter.html
http://www.astro.ucla.edu/~wright/photons_outrun.html

Last edited: Jun 16, 2007
17. Jun 16, 2007

### marcus

Good post, damgo. I like your way of explaining.