If you don't know someone's age, over time they may let out clues that tell you when they were born based on what they remember, or things they claim to have done. This can be very inaccurate. My wife said something the other day that would cause anyone to infer that she was at least ten years older than she is, but it turns out the TV show she was referring to came to her home as syndicated re-runs. (My own personal memory of the recently deceased Soupy Sales is a similar example.)
You can always ask a person his or her age, but you have to infer the age of inanimate objects like The Universe. You can use radiometric dating to find the age of a lot of rocks on Earth. Maybe there are meteors on Earth that are older. Maybe there are rocks on the Moon that are older. You can then put together the ages of the oldest rocks that you can lay your hands (and instruments) on and combine that with a theory about how the Earth and the other planets formed. This would provide a minimum estimate for the age of The Universe, providing that The Universe is older than our solar system.
Which it is.
Then, if you have a theory about how the visible universe formed and a few more facts you can provide a more accurate estimate of the age of the universe. As you probably know, that gets tricky because of "expansion." Well, it is said to be tricky, but really it is quite simple. Think, temporarily, as "distance" as a thing (Which it is, though the properties we would naturally attribute to the thing labeled by the English word "distance" do not exhaustively describe a whole thing ... distance is an aspect of spacetime. But let's set that aside here.) So expansion is this: Distance is this thing that during the history of The Universe has gotten larger. It continues to get larger, but it probably got larger really fast at some earlier time. Since light is moving along this "distance" thing, the actual (final) distance the light went is farther than it could have been at maximum speed without expansion happening. Light, which goes as fast as something can go and is the only thing that goes that fast, went faster than itself because it cheated. Much like Rosie Ruiz.
Assuming that we know the numbers, it is possible, given a "big bang" and few parameters, to estimate the age of the universe more accurately than you can estimate the age of a horse or a person using only physical evidence such as tooth wear or claims about what television show a person watched while still a kid. But it is still an estimate, not as well confirmed as if you had a birth certificate, and subject to minor perturbations depending on the exact numbers that are used to make the estimate So, additional empirical data are nice to have, and such data usually comes in the form of a galaxy or a nova (exploding star) that is really far away in spacetime.
So let's say you know a person, and can't figure out if she's in her 30s or 40s. If she blurts out a knowing reference to the TV show "Edge of Darkness" then you would guess 30s. If she makes a knowing reference to Survivors, you'd guess 40s. Oh, and you'd guess her origin to be British.
Or there were reruns.
Every once in a while The Universe blurts out a reference to something that suggests its age, and that happened recently. As recently reported in Nature, astronomers have observed a gamma ray burst from a dying star and dated this event to 13.1 billion light years away. Previously, the earliest direct empirical clue to the age of the universe was 12.9 billion light years away.
From the abstract of the paper:
Long-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy. Here we report that GRB 090423 lies at a redshift of z approximately 8.2, implying that massive stars were being produced and dying as GRBs approx630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.
The red shift is how speed relative to us is measured, and in an expanding universe model (i.e., a Big Bang model .... the cosmological phenomenon, not the hit TV series) that speed is higher for objects farther away.
So next time you look at a person and are wondering how old he or she is, glance up at the sky (which is where The Universe is) and know that you're never going to guess the answer to your unspoken question as accurately as astronomers will know the answer to the ultimate age-old question. Unless, of course, The Universe has something equivalent to re-runs.
Which it doesn't.
For a refresher on the history of The Universe watch this video:
Tanvir, N., Fox, D., Levan, A., Berger, E., Wiersema, K., Fynbo, J., Cucchiara, A., Krühler, T., Gehrels, N., Bloom, J., Greiner, J., Evans, P., Rol, E., Olivares, F., Hjorth, J., Jakobsson, P., Farihi, J., Willingale, R., Starling, R., Cenko, S., Perley, D., Maund, J., Duke, J., Wijers, R., Adamson, A., Allan, A., Bremer, M., Burrows, D., Castro-Tirado, A., Cavanagh, B., de Ugarte Postigo, A., Dopita, M., Fatkhullin, T., Fruchter, A., Foley, R., Gorosabel, J., Kennea, J., Kerr, T., Klose, S., Krimm, H., Komarova, V., Kulkarni, S., Moskvitin, A., Mundell, C., Naylor, T., Page, K., Penprase, B., Perri, M., Podsiadlowski, P., Roth, K., Rutledge, R., Sakamoto, T., Schady, P., Schmidt, B., Soderberg, A., Sollerman, J., Stephens, A., Stratta, G., Ukwatta, T., Watson, D., Westra, E., Wold, T., & Wolf, C. (2009). A γ-ray burst at a redshift of z ≈ 8.2 Nature, 461 (7268), 1254-1257 DOI: 10.1038/nature08459
Comments
Wow. This science stuff sure is complicated. Isn't it a lot easier just to read an old (relatively speaking) book and believe what it tells you there? I mean, 6000 years is close enough . . . isn't it??
Posted by: Don | October 29, 2009 11:34 AM
Since the "measuring stick" is expanding as the universe expands, why would there be any redshift? The space itself is expanding so there is no change in the wave length since the units of measure are expanding as well. Why not more red = more intervening dust?
Posted by: Jim | October 29, 2009 11:57 AM
Jim: More intervening dust = dimmer, but no change in redness (except to block out certain spectrum). The idea of using GRB is that it is sufficiently high energy to penetrate a lot of that, so we can see just what the redshift is.
For the first point, space is expanding and things are getting farther away. The light took 13.(coughcoughcouch) billion years to get here, but the farthest objects are some 70+ billion light years away. Make sense?
Posted by: Spiv | October 29, 2009 12:58 PM
Here is something which floated up in my thoughts recently. We know very little about the present universe because information comes in at the speed of light, creating the event horizon. We don't know what the sun is doing in real time, we only know what it was doing eight minutes ago. Same for a star a hundred light years away, or a galaxy a million light years away. We have no possible observation of what is going on there now. On the other hand we know a great deal about the history of the universe. We know what the sun was like eight minutes ago, etc. Anyway, it surprised me when it occurred to me.
Posted by: Jim Thomerson | October 29, 2009 1:00 PM
Jim: re why redshift?
remember the science experiment in school where you send waves along a string? Well, imagine that string.
You start moving your arm up & down at 1Hz (i.e. a complete wave every second).
That results in a wave train traveling down the string at some velocity. The speed that waves travel along the string is 'constant' in that medium. (i.e. the wave propagates at a constant speed, regardless of the frequency of the wave. If you start a wave at time t, it will reach a distant point d, at some time t-prime. The ratio of distance over time is a constant - double the distance, the wave will take exactly twice as long to get there).
Also - since the speed is constant, the wavelength (distance between peaks) is inversely proportional to the frequency of the wave. Hence we can talk synonymously about '30-centimeter waves' or 'Gigahertz' waves (light traveling at 300 million meters per second, approx. = 1000 million 30-centimeter waves per second)
Now imagine we STRETCH the string after we create a wave!
The point previously at distance 'd' is now some distance farther at 'd-prime'. Assuming* the propagation speed is unchanged (still original d/t), the wavelength of the propagated wave is therefore greater (stretched by the same factor of d-prime/d). The frequency of the propagated wave is therefore lower.
aka Redshift.
We *know* this, since we observe the redshift of known spectral lines (eg Hydrogen). The emission spectra for Hydrogen is presumed to be constant. Changes to the emission spectrum is therefore direct evidence of redshift, and therefore of expansion. see http://en.wikipedia.org/wiki/Hydrogen_line
* this assumption holds for General & Special Relativity since it is a postulate of both theories that light in a vacuum has a constant rate of propagation for all observers.
Posted by: TonyC | October 29, 2009 1:34 PM
Jim: The redshift is caused by more than one thing, actually. But space expanding is one of the things that actually does.
See this:
http://scienceblogs.com/startswithabang/2009/08/redshift_and_distance_in_the_e.php
Posted by: Greg Laden | October 29, 2009 1:48 PM
I love the TV Show metaphor, and may reuse it to explain why fossils do not tell us absolute dates for phylogenetic lineages sometime, if you don't mind.
Posted by: John S. Wilkins | October 29, 2009 1:50 PM
Incidentally, Bare Naked Ladies claim the Big Crunch is still an option. I think that's been dropped now...
Posted by: John S. Wilkins | October 29, 2009 1:53 PM
if you want more info on the age of the universe and expanding space etc go check out Starts with a Bang. it is another blog here at Science Blogs. look at the series of articles written last month on the subject.
Posted by: rob | October 29, 2009 1:55 PM
Wow.
And here I thought you were going to talk about passing gas.
Posted by: GaryB | October 29, 2009 3:41 PM
John: Use it, of course! Personally I liked it too. Works with temporally distinct musical genre as well.
Regarding the big crunch, doesn't this all come down to if we believe in Dark Matter? Which we probably do, of course.
Rob: Yeah, the link I give above is to a SWAB article. I would strongly recommend people go check it out.
Posted by: Greg Laden | October 29, 2009 5:54 PM
NASA's latest Astronomy Picture of the Day is relevant here:
http://antwrp.gsfc.nasa.gov/apod/ap091028.html
Posted by: Alex | October 29, 2009 10:48 PM
Any extra gravitational effect due to Dark Matter should be more than compensated for by Dark Energy (whatever it might be) which is causing the Universe's expansion to accelerate.
Posted by: Alex | October 30, 2009 12:03 AM
Right but ... this is like the time my daughter (when she was very little) asked me if I believed in the Easter Bunny.
"Why does it matter if the Easter Bunny is real?" I asked.
"Because if the Easter Bunny is real, it would sure make it a lot easier to believe in Santa Claus."
Anyway, I figured Dark Energy is there to offset the Dark Matter and therefore fix up all the calculations. Dark matter makes it a lot easier to believe in dark energy.
Posted by: Greg Laden | October 30, 2009 12:08 AM
Greg: Dark energy is more a means to describe a known property of the universe (accelerating expansion). This puts 'big crunch' out of contention in general. It's really a kind of placeholder (IMO, anyway) to describe whatever it is that causes this observed result.
Dark energy is most probably vacuum energy or scalar fields, neither of which we have a good grasp on. Vacuum energy would make sense in Einstein's world, and he's a bad choise to bet against.
Dark matter is same thing, different problem. Gravity points to places where apparent mass isn't, so there must be some sort of matter there that we can't properly detect. Because of where it tends to be (like, when two galaxies pass through each other the regular matter gets all tangled up and the dark matter just cruises on as if nothing happened) we can imply that it doesn't interact the same way as regular mass does. Hence the probable weakly interacting massive particles (WIMPs)
In other words: you can believe them or not, but the results are real phenomena. The math is an afterthought.
Posted by: Spiv | October 30, 2009 9:31 AM
It is my understanding that if a light object is moving away from the earth, it would have a red shift. If a light object is moving toward the earth it would have a blue shift.
A picture from the website hubblesite.org of the Omega Centauri globular cluster shows a number of lights that are red and some lights that are blue.
http://hubblesite.org/gallery/wallpaper/pr2009025q/
Could you take a look at the picture and explain why some of the lights are red and some are blue? Would it be safe to conclude that some objects in the universe are moving towards us and some objects are moving away from us?
Posted by: Jerry | November 2, 2009 9:22 AM
Jerry: Others can probably comment on this better but a star that is red shifted has a spectrum that is shifted to the left but that does not mean the star would appear red. If you look at the spectrum of a star you can see absorption lines for hydrogen. If a star is red-shifted, the absorption lines will be shifted slightly to the red end of the spectrum but especially if the star is close and the shift is very small the star would not appear red. There's a nice sample image explaining it at http://en.wikipedia.org/wiki/Redshift
The example you chose was the Centauri system which is one of the closest stellar systems to us so and red shift would be extremely small. The reason some stars appear blue while some appear red in this case simply has to do with the temperature and size of the stars themselves. Blue stars burn hotter than cooler (relatively speaking of course) red stars.
Posted by: Kirk | November 4, 2009 2:56 PM
I don't know about this particular picture, but plenty of galaxies are moving towards us. Most are not, and critically, the farther away a galasy is, the more red shifted it is, because of expansion.
Posted by: Greg Laden | November 4, 2009 5:28 PM