Age of the Universe
How old is the universe? It is a question answered for thousands of years with dreams and speculation. Only in the last 50 years have answers based on observation become possible, and even now astronomers disagree.
Such disagreement reached the public’s eyes and ears recently when several astronomers announced that the universe was not 15 billion years old, as most astronomers had believed, but only 10 billion years old. Despite the publicity given their then-unpublished results, most astronomers remain unconvinced. To understand why it is important to know how astronomers measure the universe’s age.
Asking about the age of the universe is a meaningful question because astronomers believe that the universe has not existed forever, but that it began in one unimaginably hot and dense fireball called the big bang. That our universe has a finite age is philosophically intriguing. That we can estimate that age to a fair degree of accuracy is truly impressive.
The “good” one is to think about the fact that our Universe is expanding and cooling today and to recognize that it was, therefore, hotter and denser in the past. If we go back, to earlier and earlier times, we’d find that as the volume of the Universe was smaller, all the matter in it was not only closer together, but that the wavelengths of all the individual photons (particles of light) in it were shorter, as the Universe’s expansion has lengthened them to be as long as they are today.
In physical cosmology, the age of the universe is the time elapsed since the Big Bang. The current measurement of the age of the universe is around 13.8 billion years (as of 2015) – 13.799±0.021 billion years within the Lambda-CDM concordance model. The uncertainty has been narrowed down to 20 million years, based on a number of studies which all gave extremely similar figures for the age.
These include studies of the microwave background radiation by the Planck spacecraft, the Wilkinson Microwave Anisotropy Probe and other space probes. Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang, and measurements of the expansion rate of the universe can be used to calculate its approximate age by extrapolating backwards in time.
Since a photon’s wavelength defines its energy and temperature, a shorter-wavelength photon is more energetic and higher in temperature. As we go back farther and farther in time, the temperature goes up and up, until, at some point, we reach the earliest stages of the hot Big Bang. This is important: there is an “earliest stage” for the hot Big Bang!
If we were to extrapolate “infinitely” far back, we’d reach a singularity, where physics breaks down. With our modern understanding of the very early Universe, we know that an inflationary state preceded the hot, dense Big Bang and that inflationary state was of an indeterminate duration. So when we speak of “the age of the Universe,” we’re talking about how much time has past since the Universe could first be described by the hot Big Bang until the present day.
Estimating Age of the Universe
How do you estimate age? For people, you probably look at their face and make a judgment. For a tree, you count tree rings. For a fossil, you can do carbon dating. In any case, you can see there are many different ways we can determine the age of something. But how do we determine the age of something as vast as the universe? How old is the universe anyways?
Stephen A. Naftilan, professor of physics in the Joint Science Department of the Claremont Colleges, responds:
“Astronomers usually cannot tell the age of an individual star. There are certain stars that we know are very young, and others that are very old, but for most stars, we cannot tell. When we have a large group of stars, however, we can tell its age. This is possible because all of the stars in a cluster are presumed to have begun their life at approximately the same time. After a relatively brief time (in ‘star time,’ that is–we are talking thousands to millions of years here) stars reach the adult phase of their life, which we call the main sequence phase. The length of time a star spends in the main sequence phase depends on its mass.
“Constructing a plot, called the HR diagram, of the stars in the cluster, scientists can determine the mass of the stars that are just ending this phase and moving on to the next phase of their life, the red giant phase. Computer models allow us to predict how old a star of that mass must be to be at that juncture of its life, and hence to estimate the age of the cluster. Recently, this procedure has come under close scrutiny because that age it gives for the oldest star clusters in our Milky Way seems to be older than the age of the universe derived from the most recent Hubble Space Telescope data.”
The first place to start is with the expanding Universe itself and the one-parameter we’ve strived to measure longer than any other: the Hubble constant. On the largest scales, the galaxies we find in the Universe obey a very simple relationship between the two observable quantities of distance and redshift, where the farther away an object is from us, the greater its measured redshift will be.
Remarkably, the law that relates them is extremely straightforward: the recession speed that you would infer from a galaxy’s redshift equals the distance to that galaxy multiplied by the Hubble constant. Even more remarkably, that constant has the same value for pretty much every galaxy we measure, particularly for galaxies within a few billion light-years of us. Even though there are additional cosmic motions inherent to each galaxy induced by gravitational effects, this law remains true when you average over all the galaxies you can find.
Age of the Universe – Important Equations
About 13.7 billion years ago, the Big Bang occurred. The Big Bang is an explanation of how the universe began from an infinitely compact state, and it thus marks the age of our universe. The universe has expanded ever since the Big Bang. I know that you can picture two galaxies within the universe flying apart as it expands. That’s easy to do. But we can just as easily reverse this and imagine them flying back towards one another.
Thus, we can calculate the time it would take for these galaxies to collide back to where they started out from. In a simple way, the time (T) it would take for them to collide is equal to the distance (d) divided by velocity (v). We can then modify this equation by using the Hubble law. The Hubble law is a direct (linear) relationship between a galaxy’s velocity of recession and its distance. The average value of the velocity of recession divided by distance is the Hubble constant.
The Hubble Law
If we agree that Hubble’s Law tells us that the universe is expanding, it also implies that in the past the universe was much smaller than it is today. If we assume that the expansion’s apparent velocity (that is, how fast the galaxies appear to be moving apart) has been constant over the history of the universe, we can calculate how long ago the galaxies began their separation. This should tell us the time that the expansion began, which should give us an estimate of the age of the universe.
Astronomers showed that supernovae of type Ia (see The Death of Stars), with some corrections based on the shapes of their light curves, are standard bulbs. This type of supernova occurs when a white dwarf accretes enough material from a companion star to exceed the Chandrasekhar limit and then collapses and explodes. At the time of maximum brightness, these dramatic supernovae can briefly outshine the galaxies that host them, and hence, they can be observed at very large distances. Large 8- to 10-meter telescopes can be used to obtain the spectra needed to measure the redshifts of the host galaxies
The result of painstaking, careful study of these supernovae in a range of galaxies, carried out by two groups of researchers, was published in 1998. It was shocking—and so revolutionary that their discovery received the 2011 Nobel Prize in Physics. What the researchers found was that these type Ia supernovae in distant galaxies were fainter than expected from Hubble’s law, given the measured redshifts of their host galaxies. In other words, distances estimated from the supernovae used as standard bulbs disagreed with the distances measured from the redshifts.
If the universe were decelerating, we would expect the far-away supernovae to be brighter than expected. The slowing down would have kept them closer to us. Instead, they were fainter, which at first seemed to make no sense.
Before accepting this shocking development, astronomers first explored the possibility that the supernovae might not really be as useful as standard bulbs as they thought. Perhaps the supernovae appeared too faint because dust along our line of sight to them absorbed some of their light. Or perhaps the supernovae at large distances were for some reason intrinsically less luminous than nearby supernovae of type Ia.
If the expansion of the universe is happening rapidly, then we expect the universe to be relatively young, because it has taken only a short time for the galaxies to expand to large distances. If, on the other hand, the universal expansion is progressing at a slow speed, then the age of the universe should be relatively old, because it has taken a long time for the galaxies to reach large distances from each other. We know how fast the universe is expanding because we know the value of Hubble’s constant (H0 ). The faster the universe is expanding, the faster the galaxies will appear to be moving away from each other.
You can actually calculate an estimate for the age of the Universe from Hubble’s Law. The distance between two galaxies is D. The apparent velocity with which they are separating from each other is v. At some point, the galaxies were touching, and we can consider that time the moment of the Big Bang. If you take the separation between the two galaxies (D) and divide that by the apparent velocity (v), that will leave you with how long it took for the galaxies to reach their current separation. The standard analogy here is to consider that you are now 300 miles from home. You drove 60 mph the entire time, so how long did it take you to get here? Well, 300 miles / 60 mph = 5 hours.
- So, the time it has taken for the galaxies to reach their current separations is t=D/v.
- But, from Hubble’s Law, we know that v=H0D.
- So, t=D/v=D/(H0×D)=1/H0 . So, you can take 1/H0 as an estimate for the age of the Universe.
- The best estimate for H0=73km/s/Mpc. To turn this into an age, we’ll have to do unit conversion.
- Since 1Mpc=3.08×1019km , H0 = (73 km/s/Mpc) x (1 Mpc/3.08 x 1019 km) = 2.37 x 10−18 1/s .
- So, the age of the Universe is t = 1/H0 = 1 / 2.37 x 10−18 1/s = 4.22 x 1017 s = 13.4 billion years .
From stellar evolution, we have estimated the ages of the oldest globular clusters to be approximately 12-13 billion years old. These are the oldest objects we have identified, and it is a nice check on our estimates for the age of the Universe that they are consistent. It would have been strange if we were unable to find any objects roughly as old as the Universe or if we found anything significantly older than the estimated age of the Universe.
For many years, until about 10 years ago, however, there was a controversy over the age of the universe derived from Hubble’s Constant. The best theories available at the time were estimating that the stars at the Main Sequence Turn Off in many globular clusters had ages of 15 billion years old or more. This creates a problem.
How can the universe contain an object older than itself? Recently, however, advances in our understanding of the stars have led us to refine the ages of the stars in globular clusters, and we now estimate them to be about 13 billion years old. This means, though, that the stars in the globular clusters must have formed within the first several hundred million years of the universe’s existence!
WMAP
NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) project’s nine-year data release in 2012 estimated the age of the universe to be (13.772±0.059)×109 years (13.772 billion years, with an uncertainty of plus or minus 59 million years).
However, this age is based on the assumption that the project’s underlying model is correct; other methods of estimating the age of the universe could give different ages. Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude.
This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of the universe at the time of recombination). The light travel time to this surface (depending on the geometry used) yields a reliable age for the universe. Assuming the validity of the models used to determine this age, the residual accuracy yields a margin of error near one percent.
To determine the density and composition of the universe, scientists rely on missions such as NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and The European Space Agency’s Planck spacecraft. By measuring the thermal radiation left over from the Big Bang, missions such as these are able to determine the density, composition and expansion rate of the universe. The leftover radiation is known as the cosmic microwave background, and both WMAP and Planck have mapped it.
In 2012, WMAP estimated the age of the universe to be 13.772 billion years, with an uncertainty of 59 million years. In 2013, Planck measured the age of the universe at 13.82 billion years. Both of these falls within the lower limit of 11 billion years independently derived from the globular clusters and both have smaller uncertainties than that number.
In 2015, the Planck Collaboration estimated the age of the universe to be 13.813±0.038 billion years, slightly higher but within the uncertainties of the earlier number derived from the WMAP data. By combining the Planck data with external data, the best-combined estimate of the age of the universe is (13.799±0.021)×109 years old
Ages of 12 billion years and up are very common, but ages of, say, 14 billion years and over are unheard of, although there was a period in the 1990s where ages of 14–16 billion years were often cited. (An improved understanding of stars and their evolution has bumped these numbers down.)
NASA’s Spitzer Space Telescope has also contributed to narrowing down the age of the universe by reducing the uncertainty of the Hubble constant. Combined with the WMAP measurements, scientists were able to make independent calculations of the pull of dark energy.
“Just over a decade ago, using the words ‘precision’ and ‘cosmology’ in the same sentence was not possible, and the size and age of the universe were not known to better than a factor of two,” Wendy Freedman of the Observatories of the Carnegie Institution for Science in Pasadena, California, said in a statement. Freedman leads the study that used Spitzer to refine the Hubble constant. “Now we are talking about the accuracies of a few per cent. It is quite extraordinary.”
Bottom Line
So all in all, we have two methods — one from our cosmic history and one from measuring local stars — that show us our Universe’s age is between 13 and 14 billion years old. It wouldn’t surprise anyone if we turned out to be as little as 13.6 or as much as 14.0 billion years old, or maybe even as little as 13.5 or as much as 14.1 billion. But we’re not 13.0 or 15.0 billion years old, and we’ve determined that with extreme certainty. Say we’re 13.8 billion years old with confidence, and now you know how we’ve figured it out!
The reason that we can claim the Universe is 13.8 billion years old to such enormous precision is driven by the full suite of data that we have. A Universe that expands more quickly needs to have less matter and more dark energy, and its Hubble constant multiplied by the age of the Universe will have a larger value. A slower-expanding Universe requires more matter and less dark energy, and its Hubble constant multiplied by the age of the Universe gets a smaller value.
However, in order to be consistent with what we observe, the Universe can be no younger than 13.6 billion years and no older than 14.0 billion years, to more than 95% confidence. There are many properties of the Universe that are indeed in doubt, but its age isn’t one of them. Just make sure you take the Universe’s composition into account, or you’ll wind up with a naive — and incorrect — answer.
