The Universe and the Earth's age

[*Quinn*]

I just caught the end of some show on the History channel and caught them saying the Universe is something like 12.5 billion years old and that Earth is roughly 4.5 billion years old.

How do they figure out something like that? I've heard before about Earth being that old (still don't know how they determine it though), but I never really heard them put an age on the Universe before.

I didn't catch much of the show (just the last 5 minutes or so), but it seemed it was discussing the possibility of life elsewhere in the Universe outside of planet Earth. Given the age of the Universe in comparison to Earth, the show also made the comment that if life exists on other planets they are likely to be billions of years more advanced then us due to the age of the Universe and the belief that these planets in other solar systems would be billions of years older than Earth.

I've never heard of it being put that way before and I'm just wondering if anyone knows how scientists come to these conclusions. Any ideas?

[Torgo]

Its actually pretty fascinating! I'm taking an astrophysics course and the way the universe is dated is pretty cool.

As for the universe: it's all about light and distance. This can get a little complicated... firstly, we can tell the distances to relatively nearby stars directly, from parallax (the stars moving against the background of much more distant stars over the course of the year as we move in our orbit). In addition, just about all stars of a given color will put out a very similar amount of light, so we can look at the spectrum of a star and compare its brightness and see how far away it must be in order to appear that dim. In addition, there are certain star types that A) vary their brightness with a regular period and B ) have a very precise average brightness proportional to that period. All of these things let us tell the distances to a huge number of RELATIVELY nearby galaxies - and the distances we get are in the hundreds of millions to billions of lightyears. We also notice something - with pretty good regularity, the further away a galaxy is from us, the faster it is receding from us and the more redshifted its light is. This indicates that everything is moving away from everything else, and it also lets us use the redshift of light from far away galaxies to tell their distance and thus, since light has a constant speed, how long ago the light was emitted.

We notice that the further away we look and thus the further back we look in time the denser things are. We see galaxy types long ago and far away that simply don't exist now. The furthest galaxies we can't discern individual stars in, but we can look at their degree of redshift and tell how far away and long ago they are. This combined with the fact that the universe appears to be expanding suggests that long ago it was denser and hotter (since when you squeeze a gas it heats up).

Behind all these galaxies, coming pretty darn uniformly from all directions in the sky, is a background glow of radiation (mostly in the microwaves). This radiation is special - its spectrum is SHAPED like the spectrum given off by a very hot gas, but it is not high enough frequency for that - instead of ultraviolet, it is microwaves. If you look at the redshift required to take the radiation emitted from a hot gas and make it into what we see, and correlate that redshift to the time the light has been traveling, you get a value of 13.7 billion years. If you run the models of universe expansion backwards, at that point things get so hot the gas turns to plasma and light can't travel through it. You keep running them back, the universe reaches zero size 300,000 years before this moment. So we can say with pretty good certainty that the universe as we know it is about 13.7 billion years old.

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Dating the Earth is actually quite a bit simpler. It has to do with radioactive decay, mostly. As I'm sure you're aware, certain unstable forms of certain elements decay into other elements with a fixed half life. We use minerals that sequester one element inside it but not any of the elements it decays into. For example, there is one isotope of uranium with a well established halflife of a a few billion years. When a certain mineral forms from molten rock, crystals containing uranium but that exclude any and all lead form inside it. Over time, some of the radioactive uranium decays into lead and this lead is trapped inside the crystal with no way out. By looking at the ratio of different uranium isotopes and lead inside the crystal, you can determine how long it has been since the rock the crystal is in formed from magma. There are many independent radioactive clocks using different isotopes, and they all agree pretty well.

The very oldest rocks we find on Earth's surface are 3.9 billion years old - these are in the middle of continents, far away from volcanoes and faults. New rock is constantly being formed and old rocks are constantly being recycled, so most rocks on Earth's surface are quite a bit younger. However, when you look at METEORS that fall to Earth from outer space, almost ALL of them show the exact same age - 4.56 billion years. This suggests that the solar system formed about then, or at least small rocks like meteors formed then. The Earth was probably not totally solid until the 3.9 BYA figure, or geological activity may have erased all signs of slightly older rocks.

There are other corroborating lines of evidence, having to do with cratering rates and the rates at which moons move towards/away from planets from tidal effects and how fast planets of different sizes cool - and they all point towards the same age of the solar system. Certain pieces can be quite a bit younger - things still hit each other. But overall, 4.56 billion years is the accepted age.

Edited February 13, 2008 by Torgo

[chrisfreak]

Wow.. amazing! Thanks Torgo!

[*Quinn*]

I will read that a little later torgo...thanks for the response

and oh yea, Mystery Science Theater 3000 (or 2000, whatever it is) always cracked me up. I don't see it on anymore though, but I guess I should be staying on topic.

[8th_wall]

Can somebody explain to me in layman's terms why we can fiew things 30 billion light years away but the universe is onoly 16 billion years old? How could light in the universe have been travelling for 30 billion years and us able to see it if the universe is 16 billion years old?

[Waspie_Dwarf]

Can somebody explain to me in layman's terms why we can fiew things 30 billion light years away but the universe is onoly 16 billion years old?

The short answer is that we can't. The furthest detected objects are young galaxies around 13 billion light years away.

[Torgo]

MST3K is AWESOME! Anyways:

Can somebody explain to me in layman's terms why we can fiew things 30 billion light years away but the universe is onoly 16 billion years old? How could light in the universe have been travelling for 30 billion years and us able to see it if the universe is 16 billion years old?

Yeah... this is where it gets complicated and wonky to explain.

There is a difference between travel time of the light and the distance calculated. This comes from a few things. If you consider the galaxies moving away from us at very nearly the speed of light (which the furthest ones are according to their spectrums) , the standard doppler shift isn't the whole story. You have to include the RELATIVISTIC doppler shift - basically the time dilation effect causes the light to shift towards redder in addition to the motion away shifting it. Relativity makes everything VERY wonky... to the point that you can have from one point of view two galaxies moving away from you in opposite directions at nearly the speed of light, but if you shift your reference frame they can appear to have nearly no relative velocity just both moving in the same direction at very slightly different speeds near the speed of light. I can explain this a bit better if people are still interested, it can get very complicated.

In short, because of relativity distances and velocities change depending on your frame of reference and you need to do some corrections to get the light travel time.

If you use the interpretation that the space between the galaxies is expanding, the distances calculated by the redshifts are actually real. In that case, these galaxies would actually be the distance indicated by the redshift calculation away - but the travel time of the light would be a lot less, since the light was emitted when they were a lot closer and they have moved off significantly as the intervening space has gotten larger. This is not inconsistant with the relativity explaination, as far as I can tell, and seems to be the widely accepted interpretation of the data.

You get the figure that the furthest places away we can see are now about 40-50 billion lightyears away (something like that, can't remember the figure) and that the light has taken 13.7 billion years to reach us.

I may have misworded a few things here, but this is the best I can explain it to the best of my knowledge. I'm not an astronomer and i COULD be wrong as to an interpretation or something like that. However, what matters is that we can tell the universe is about 13.7 billion years old (the 16 billion years came from an older measurement that wasn't as precise as the ones we can make now. Our current measurement is 13.7 +/- 0.2 billion years, so don't be surprised if in the future they refine the value to somewhere between 13.5 and 13.9 billion years).

Edited February 14, 2008 by Torgo

[8th_wall]

Oh okay, I understand. The light that took 14 billion years to reach us from an object that was 14 billion light years away at that point in space relative to the point in space we are at now has a new actual position of 50 billion light years away at the point of us observing it after doing some calculations via the red spectrum shift from the light ommited to figure out its velocity. Did I get the basics right?

Edited February 15, 2008 by PsiSeeker

[Torgo]

Basically yeah. I did some more research and apparently there are named mathematical models that are used to figure out ages and distances from redshifts.

[8th_wall]

Basically yeah. I did some more research and apparently there are named mathematical models that are used to figure out ages and distances from redshifts.

Wow, very interesting, thanks dude.

[Truffles]

Its actually pretty fascinating! I'm taking an astrophysics course and the way the universe is dated is pretty cool.

As for the universe: it's all about light and distance. This can get a little complicated... firstly, we can tell the distances to relatively nearby stars directly, from parallax (the stars moving against the background of much more distant stars over the course of the year as we move in our orbit). In addition, just about all stars of a given color will put out a very similar amount of light, so we can look at the spectrum of a star and compare its brightness and see how far away it must be in order to appear that dim. In addition, there are certain star types that A) vary their brightness with a regular period and B ) have a very precise average brightness proportional to that period. All of these things let us tell the distances to a huge number of RELATIVELY nearby galaxies - and the distances we get are in the hundreds of millions to billions of lightyears. We also notice something - with pretty good regularity, the further away a galaxy is from us, the faster it is receding from us and the more redshifted its light is. This indicates that everything is moving away from everything else, and it also lets us use the redshift of light from far away galaxies to tell their distance and thus, since light has a constant speed, how long ago the light was emitted.

We notice that the further away we look and thus the further back we look in time the denser things are. We see galaxy types long ago and far away that simply don't exist now. The furthest galaxies we can't discern individual stars in, but we can look at their degree of redshift and tell how far away and long ago they are. This combined with the fact that the universe appears to be expanding suggests that long ago it was denser and hotter (since when you squeeze a gas it heats up).

Behind all these galaxies, coming pretty darn uniformly from all directions in the sky, is a background glow of radiation (mostly in the microwaves). This radiation is special - its spectrum is SHAPED like the spectrum given off by a very hot gas, but it is not high enough frequency for that - instead of ultraviolet, it is microwaves. If you look at the redshift required to take the radiation emitted from a hot gas and make it into what we see, and correlate that redshift to the time the light has been traveling, you get a value of 13.7 billion years. If you run the models of universe expansion backwards, at that point things get so hot the gas turns to plasma and light can't travel through it. You keep running them back, the universe reaches zero size 300,000 years before this moment. So we can say with pretty good certainty that the universe as we know it is about 13.7 billion years old.

------------------------------------------------------------------------------------------------

Dating the Earth is actually quite a bit simpler. It has to do with radioactive decay, mostly. As I'm sure you're aware, certain unstable forms of certain elements decay into other elements with a fixed half life. We use minerals that sequester one element inside it but not any of the elements it decays into. For example, there is one isotope of uranium with a well established halflife of a a few billion years. When a certain mineral forms from molten rock, crystals containing uranium but that exclude any and all lead form inside it. Over time, some of the radioactive uranium decays into lead and this lead is trapped inside the crystal with no way out. By looking at the ratio of different uranium isotopes and lead inside the crystal, you can determine how long it has been since the rock the crystal is in formed from magma. There are many independent radioactive clocks using different isotopes, and they all agree pretty well.

The very oldest rocks we find on Earth's surface are 3.9 billion years old - these are in the middle of continents, far away from volcanoes and faults. New rock is constantly being formed and old rocks are constantly being recycled, so most rocks on Earth's surface are quite a bit younger. However, when you look at METEORS that fall to Earth from outer space, almost ALL of them show the exact same age - 4.56 billion years. This suggests that the solar system formed about then, or at least small rocks like meteors formed then. The Earth was probably not totally solid until the 3.9 BYA figure, or geological activity may have erased all signs of slightly older rocks.

There are other corroborating lines of evidence, having to do with cratering rates and the rates at which moons move towards/away from planets from tidal effects and how fast planets of different sizes cool - and they all point towards the same age of the solar system. Certain pieces can be quite a bit younger - things still hit each other. But overall, 4.56 billion years is the accepted age.

Thats amazing info. I think I understood some of it. lol. I heard the universe is constatly expanding. I don't know if that means every minute or year but if it is and its THAT old IMAGINE the size. I can't fathom it. It's so fascinating!