The Age of the Universe

This is quite a long read but that is both appropriate and relative because the Universe’s lifespan is also long, I mean LOOOOONG. Never has the phrase “as old as time” been quite so appropriate as it is when talking about the age of Universe. It would be fair to say the Universe is long past trying to fool people by only putting 49 candles on it’s birthday cake. If you want the short answer, the Universe is about 14 billion years old, give or take a few billion years. If you want the long answer then look no further because below is a *concise* summary of how we know the age of the universe.

The concept of the Universe having an age is actually a relatively new concept.  During the first half of the twentieth century the prevailing scientific opinion was that the universe was constant and eternal, it was in a steady state. It always would be and always will be. However, this theory struggled to explain the discovery that all the stars and galaxies where moving away from us. The latter half of the century saw the emergence of the big bang theory and the idea of an expanding universe where all the galaxies are moving away from each other. The term “big bang” was actually coined by Fred Hoyle who was a staunch steady state man because the name reflected his view that the big bang was just “cartoon physics”. Now I’m not going to mention Hoyle again but don’t let this colour your opinion of him too much for he was by all accounts an incredibly gifted physicist, just stubborn and unwilling to accept this newfangled big bang theory.

Returning to this expanding Universe idea, if all the galaxies are moving away from each other, it would mean that in the past the galaxies were closer together. If we rewind all the way back, at some point in the past all the galaxies must have come from the same point, this point was the big bang. It is this that we state as being the start of the universe. Before the big bang there was no space, no time and nothing that could have any influence on events after the big bang. So, when we ask “what is the age of the universe?” we are also asking; what is the age of time itself?

Lower Limits and the Age of the Earth

When determining the age of the universe, lower limits can be determined from the basic assertion that the universe cannot be younger than the objects it contains, a parent must be older than their child. Our current understanding of how stuff in the universe was made suggests that the age of the Earth ought not be too close to that of the universe’s age, the Earth should be much younger, but it represents an lower limit. The age of the Earth is also determined by lower limits. The Earth must be older than any of the rocks that make it up. Since Earth’s geological activity constantly destroys and makes rocks it is unlikely that any rock found would be from the first generation of rocks and so more than likely any rock we find was made after the formation of the Earth. The oldest rock discovered was found to have an age of 4.4 billion years.

How do we find the age of such a rock? I’m glad you asked although you may not be! Zircon, the material from which the dated rock is made, contains zirconium, silicon, and oxygen however due to the similarities in the properties of zirconium and uranium, occasionally a stray uranium atom replaces the zirconium in the rocks lattice. Unlike zirconium, uranium is radioactive which means it will eventually decay into lead. Lead and zirconium do not have similar properties therefore the only way for lead to be found in zircon is for the uranium to have decayed. The amount of lead in the zircon can be used to determine its age, the more lead, the older the Zircon. Why not give it a go with a grandparent or great aunt?

Since the rock is 4.4 billion years old, the Earth must also be at least 4.4 billion years old. This can also be used as a lower limit for the age of the universe. The universe must be at least 4.4 billion years old however we know that the universe must have been around long before the Earth since the Earth contains heavy elements. Heavy elements (really anything that isn’t hydrogen or helium and maybe lithium or beryllium) are produced almost exclusively in the life and death of giant stars, in supernovae. So before the Earth was formed there must have been at least time for a huge star to form, live at least a few million years and then die in a massive explosion spewing all our favourite heavy elements into space from which Earth then formed. This does not drastically alter our lower limit, because the most massive stars have lifetimes as short as 3 million years. So our 4.4 billion year firm lower limit becomes 4.403 billion years.

Aging Globular Clusters

The Earth and even the Sun are by no means the oldest objects in the universe, nor are the dead stars from whose remains they formed. Among the oldest objects, we know of, in our universe are globular clusters. Globular clusters are huge balls of stars that all formed at roughly the same time from the same cloud of gas. This means we know that the age, composition, and distance from us, of all the stars in the cluster is roughly the same. By looking at the types of stars in the cluster we can work out the age of the cluster.

To do this we use something called a Hertzsprung-Russel diagram which places stars on the diagram according to their temperature/colour and their brightness. A little while after the formation of a cluster when all the stars are happily in the stable main stage of their life (like our Sun is now) all the stars form a diagonal line on a HR diagram, this line has the imaginative name of “the main sequence line”. The smaller stars are towards the bottom right of the line and the big stars are on the left. As the cluster ages, the more massive, brighter stars use up their fuel and enter the later stages of their lives and so move off the main sequence line. The main sequence life-time of a star is shorter, the bigger the star. So, the length of the main sequence line for the cluster shortens from the end of massive hot stars as the cluster age. Therefore, if we make a Hertzsprung-Russel Diagram for a cluster, it’s age can be determined by the turn-off point on the main sequence line. From analysis of this type, the oldest globular clusters have been found to have formed between 11 and 18 billion years ago, it’s quite hard to be precise. Therefore, the universe must be at least 11 billion years old.

Radiometric Dating of the Oldest Stars

Now we have an approximate estimate for the age of the oldest stars we can find a more accurate age for specific individual stars which we think are the oldest using radiometric methods similar to those used to date rocks on Earth. We know the ratio of elements that would have been in a star after formation. The radioactive elements would decay meaning the amount of the radioactive elements in old stars is lower today than when they were formed but the amount of non-radioactive elements is the same. So we can use the known proportions of elements in the star from current observations to calculate how much of a radioactive element there would have been when the star was formed. A comparison between the amount of radioactive material, which decays at a known rate, found in the star in the present day with the amount present when the star formed allows the age of the star to be calculated.  To get an accurate estimate of the age, the radioactive elements we measure should have an average decay time (or half-life) about the same as the age of the stars, and therefore about the same as the age of the universe. We know that the age of the universe is about 10-20 billion years therefore the concentrations of Uranium and Thorium with half-lives of 4.5 and 14.1 billon years respectively are analysed. Analysing the thorium, the oldest stars are found to be 15.6 billion years old give or take 5 billion years. When analysing the Uranium, the oldest stars are found to be 14.1 billion years old give or take 2.5 billion years. The Uranium gives results that are more accurate and that better agree with the current theoretical age in cosmology.

The Cosmological Model

The theoretical age of the universe is calculated by extrapolating the expansion of the universe back to the big bang. The precise moment of the big bang is not well defined and is not well explained by physics. Only one microsecond after the big bang is, by contrast, well defined. So, we could refer to the age calculated as the age of the universe minus one microsecond but due to the large uncertainties involved, the one microsecond fades into insignificance. The theoretical age of the universe depends upon something called the Hubble parameter, H0, and how it has changed over time. The Friedman equations determine how the Hubble Parameter has changed over time. To calculate a precise age of the universe in this way requires accurate measurement of four parameters; the Hubble Parameter (H0), the radiation density of the universe (Ωr), the matter (including dark matter) density of the universe (Ωm) and a cosmological constant (ΩΛ). The age of the universe is largely dependent on the value of H0, it is inversely proportional. The proportionality factor depends upon the other three parameters but is approximately 1. The accuracy with which we can measure the four parameters determines how well we can estimate the age of the universe. If you’re feeling a little giddy don’y worry you don’t need to know what all of this, I just include it for those that might care. Long story short, the universe is expanding. If we know how fast it’s expanding now (and how fast it expanded in the past) we can work out how long it must have taken to get to it’s current size. We know how fast it’s expanding by finding the value of H0.

 H0 is determined using Hubble’s Law. Hubble’s Law states that the faster galaxies are moving away from us, the further away they are from us. The constant that defines this relationship is H0. Therefore, the accuracy of our value of H0 depends upon the precision with which we can measure the velocities and the distances of distant galaxies. The velocities are calculated by analysing the redshift of the galaxies and as such we can measure this to a high degree of accuracy. You may have come across redshift before, in the same way the pitch of an ambulance siren changes as it drives past you, the colour objects emit changes as the move away or towards you (if they are moving fast enough). If they move away from you they look slightly red.

The distance to the galaxies cannot be measured directly so a cosmic distance ladder is used. That is to say we work up in steps using the distance of things we know. Using parallax measurements, the distance to nearby Cephid variables (a type of pulsing star) can be determined. Cephid variables are used because their brightness is reliable to predict. A cephid variable can then be observed in another galaxy, where parallax would not be able to determine its distance, and we can be determine its distance from us by comparing it’s brightness to the ones close to us, and thus work out the distance to the galaxy it is in. Within these nearby galaxies, type 1a supernovae are observed. Type 1a supernovae are useful because they can, like all supernovae, rival the brightness of the entire galaxy they reside in and as such can be observed in very distant galaxies. Moreover, due to the nature of a type 1a supernovae (type 1a supernovas happen when the mass of a white dwarf accreting matter from a companion star exceeds the Chandrasekhar limit, therefore all Type 1a supernovas occur from a star of similar mass and temperature) their brightness’ are all roughly the same due to near identical initial conditions. Then much as before, the supernovae are observed in distant galaxies and the distance can be determined. It is in this way the distance and velocity of a great number of galaxies are measured using this kind of distance stepping stone method to work our way to large distances. Then we can use this information to find H0. The WMAP (a very swanky NASA satellite telescope) determined the value of Hubble’s Constant and found the age of the universe to be about 14 billion years old.

Now we could get into a load of nonsense about how the geometry of space-time affects the value of this estimate. However, I think perhaps you can’t take anymore so here’s the gist of it. The geometry can be flat, open or closed. If our universe were closed then the telescopes prediction wold be 9.3 billion years, less than the age of the oldest stars – the Universe is then not closed (unless we’ve done something horribly wrong with our star ageing). Luckily we are pretty sure the Universe is flat. Using this information we get an age of 13.7 billion years give or take a 100 million years. This matched very nicely with our other estimates so we can be pretty sure it’s right, happy days! Now there’s still much arguing and posturing about exactly how flat the universe is and lots of other things about it’s nature but all of these changes would only add or subtract a few hundred million years to the age of the Universe so we can say with relative certainty it’s roughly 14 billion years old.

Conclusion

Ultimately, the is no direct way to measure the age of the universe. We can narrow down the bounds of error on our estimates however they all rely of our underlying theories of physics being correct. The Freidman equations are derived from General Relativity which, given its lack of cohesion with quantum mechanics, could well be proved incorrect or at least inaccurate. Moreover, our theories of stellar evolution might be incorrect as given the vast time scales involved we cannot observe the whole life of a star directly. However, using various methods we get a consistent age of the universe which indicates that it is likely to be correct. The best estimate for the age of the universe, using our current understanding of physics, is from the WMAP and that states that the universe is 13.71 billion years old and that’s good enough for me.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

Blog at WordPress.com.

Up ↑

%d bloggers like this: