To explore the history of the universe, will follow the same path that astronomers have followed in the past, starting with studies of the near universe, through exploring objects increasingly distant, and looking into the past.
The realization that the universe is changing over time came in the 1920s and 1930s when measurements of the redshifts of a large number of galaxies became available. In hindsight, it’s surprising that scientists were so shocked when they discovered that the universe was expanding. Gravity demands that the universe either expand or contract. To show what mean, let’s start with a universe of finite size, let’s say a giant sphere made up of a thousand galaxies. All of these galaxies attract each other due to their gravitation. Initially stationary, they would inevitably approach and eventually collide. The only way to avoid this collapse was if, for some reason, they were moving away from each other at high speed. Sufficient speed can avoid falling back to earth.
The problem of what happens in an infinite universe is more difficult to solve, but Einstein (and others) used their general theory of relativity to show that even infinite universes cannot be static. . At that time astronomers did not yet know that the universe was expanding (and Einstein himself was philosophically unwilling to accept a universe in motion), he changed his equations by introducing a new arbitrary term (we could call it a fudge factor), the cosmological constant. This constant represented a hypothetical repulsive force that could balance gravitational attraction on the largest scales and allow galaxies to stay at fixed distances from one another. In that way, universe could re-main still.
About a decade later, Hubble and his coworkers reported that the universe was expanding so no mysterious balancing force was required. Einstein is said to have said that the introduction of the cosmological constant was “the biggest mistake of my life”. however, relatively new observations suggest that the expansion is accelerating. Whether this acceleration is compatible with a cosmological constant is now observed. In the end it can turn out that Einstein was right after all.
Hubble Time
If we had a movie about the expanding universe and ran the movie backwards, what would we see? In our movie, instead of moving apart, the galaxies would move together and get closer and closer all the time. The matter that we can see today was once concentrated in an infinitesimally small volume. Astronomers identify this time as the beginning of the universe. The explosion of this concentrated universe at the beginning of time is called the Big Bang (not a bad term as there is no bigger explosion than the one that creates the entire universe), but when did that explosion take place?
We can reasonably estimate the time that has elapsed since the beginning of universal expansion. To see how astronomers do this, let’s start with an analogy. For example, suppose your astronomy class decides to have a party (kind of “big bang”) at someone’s home. At the end of the semester, everyone celebrates so enthusiastically that the neighbors call the police who arrive and everyone leaves the same time. You come home at 2 a.m. and discover that you forgot to check your watch to see when the police were arriving. But you measure with a map that the distance between the party and your house is 40 kilometers, and you also remember that you drove the entire trip at a constant speed of 80 km / h (as you feared the police cars Would follow you). Therefore the trip must have taken place:
time = distance/velocity = 40 kilometers/80 kilometers/hour = 0.5 hours
So the party must have broken up at 1:30 a.m.
When the universe began there weren’t any people looking at their watches, but we can use the same technique to estimate when the galaxies began to drift apart (remember, space is actually expanding, not the Galaxies expansion, movement through static space.) If we can measure how far away the galaxies are now and how fast they are moving, we can calculate how long the journey took.
Let us call the age of the universe measured in this way T0. Let us first make a simple case, assuming that the expansion has been constant since the universe began to expand. In this case, the time it took for a galaxy to move. a distance, d, from the Milky Way (remember that the galaxies were all together in a very small volume in the beginning) (as in our example)
T0 = d/v
where v is the speed velocity of the galaxy. If we can measure the speed at which the galaxies are moving away, and also the distances between them, we can tell how long ago it was began.
Such measurements should sound very familiar. Hubble and many astronomers after him had to establish Hubble’s law and the Hubble constant. We have learned in Galaxies that the distance of galaxies and their speed in the expanding universe are related by
V = H × d
where H is the Hubble constant. Combining these two expressions gives us
T0 = d/v= d/(H × d) = 1/H
So see that the calculation of this time was already done for us when astronomers measured the Hubble constant. The estimated age of the universe turns out to be the reciprocal of the Hubble constant (i.e. 1 / H). This age estimate is sometimes referred to as Hubble time. For a Hubble constant of 20 kilometers / second per million light years, the Hubble time is approximately 15 billion years. The unit used by astronomers for the Hubble constant is kilometers / second per million parsecs. Units, the HubbleConstant corresponds to about 70 kilometers / second per million parsecs, again with an uncertainty of about 5%.
To make the numbers easier to remember, actually rounded the Hubble constant estimates a little closer to 21 or 22 kilometers / second per million light years, which would bring the age closer to 14 billion years, but it’s still around 5%. Uncertainty in the Hubble constant, which means that the estimated age of the universe is also about 5% uncertain. However, to put these uncertainties in perspective, you should know that 50 years ago the uncertainty was a factor of 2 to accurately pinpoint Hubble’s constant over the past two decades.
Role Of Deceleration
Hubble time is only the correct age for the universe if the rate of expansion has been constant over time since the universe began to expand. Continuing our end-of-semester party analogy, this assumes you traveled home from the party for a constant rate. when in reality this was not the case. At first you might have been crazy about having to go fast, but when you calmed down and thought of police cars on the road, you may have started to slow down until they were going at a social limited speed (e.g. 80 km / h). Since he drove faster in the beginning, it would have taken him less than half an hour to get home in this case.
When calculating the Hubble time, we also assumed that H was constant throughout. It turns out that’s not a good assumption. When you think about it, astronomers initially expected the rate of expansion to slow down. We know that matter creates gravity, so all objects pull on all other objects. The mutual attraction between galaxies should slow expansion over time. That means, if gravity was the only force acting (a big yes, as seen in the next section), thenthe expansion must have been faster in the past than it is today. In that case, we would say that the universe slowed down, since from the beginning.
How much it was slowed down depends on the importance of gravity in slowing down the expansion. If the universe were almost empty, gravity would play a minor role. Then the slowdown would be close to zero and the universe would have expanded at a constant rate. But in a universe with a significant density of matter, gravitational pull means that the rate of expansion should now be slower than it used to be. If we use the current rate of expansion to estimate how long it will take galaxies to reach their current level. We’re going to overestimate the age of the universe, just as we may have overestimated the time it took you to get homefrom the party.
Universal Acceleration
Astronomers spent several decades looking for evidence of a slowdown in expansion, but they were unsuccessful. What they needed were 1) larger telescopes to measure the redshifts of more distant galaxies and 2) a very bright standard lightbulb (or standard candle), an astronomical object with a known luminosity that produces an enormous amount of energy and it can travel at distances of a billion light years or more can be observed.
If we compare how bright a standard bulb should be and how faint it looks in our telescopes, we can calculate its distance from the difference. It can tell us how fast it is moving in the universe so that we can measure its distance and movement independently.
These two requirements were finally met in the 1990s. Astronomers showed that supernovae of Type la (see Death of the Stars) are standard bulbs with some corrections based on the shape of their light curves. Supernova occurs when a white dwarf gets enough material from a companion star that crosses the Chandrasekhar limit and then collapses and explodes. At maximum brightness, these dramatic supernovae can briefly outshine their galaxies that host them and can therefore be observed at very great distances. Large 8-10 meter telescopes can be used to obtain the spectra necessary to measure the redshifts of host galaxies.
same galaxies either before or after the supernovae exploded.
(credit: modification of work by NASA, ESA, and A. Riess (STScI))
The result of a meticulous and careful study of these supernovae in a variety of galaxies, carried out by two research groups, was published in 1998. It was shocking and so revolutionary that his discovery won the 2011 Nobel Prize in Physics – distant galaxies were smaller than Hubble’s law expected, given the measured redshifts of their host galaxies. In other words, the estimated distances from the supernovae used as standard bulbs did not match the distances measured from the redshifts.
If the universe slowed down, we would expect distant supernovae to be brighter than expected. The slowdown would have kept them closer to us; instead they were weaker, which at first seemed to make us feel no-sense.
Before accepting this shocking development, astronomers first investigated the possibility that the supernova wasn’t really as useful as traditional lightbulbs as they thought. Perhaps the supernovae seemed too faint because the dust along our line of sight absorbed some of their light. Or maybe supernovae. at great distances, for some reason, they were inherently less luminous than nearby supernovae of type la.
A series of closer observations ruled out these possibilities. Scientists had to consider the alternative that the estimated distance from the redshift was wrong. The distances derived from the redshifts assume that the Hubble constant was truly constant for all-time. We have seen that it may not be constant for expansion to slow down. But assume that none of the assumptions is correct.
Suppose the universe is accelerating. If the universe is expanding faster today than it was billions of years ago, our motion away from distant supernovae has accelerated and moved further away from them since the explosion. Light of explosion has to travel a greater distance to reach us than if the rate of expansion were constant. The further the light travels, fainter it appears. This conclusion would naturally explain supernova observations, and this has now been corroborated by many additional observations over the past year. a couple of decades.It really does seem that the expansion of the universe is accelerating, an notion so unexpected that astronomers initially hesitated to think about it.
How can the expansion of the universe be accelerated? If you want to speed up your car, you need to secure the power supply by accelerating. Energy must also be added to accelerate the expansion of the universe. The discovery of the acceleration was shocking because scientists still have no idea what the energy source is. Scientists call what-ever it is dark energy, which is a clear indication of how little we understand it.
Note that this new component of the universe is not the dark matter. Dark energy is another thing that we have not yet discovered in our laboratories on Earth.
What is dark energy? One possibility is that it is the cosmological constant, which is an energy associated with the vaccum of “empty” space itself. Quantum mechanics (the fascinating theory of how things behave at the atomic and subatomic levels) tells us that the source of this vacuum energy could be tiny elementary particles that flicker in and out of existence every-where throughout the universe. Several attempts have been made to calculate how great the effects of this vacuum energy should be, but so far these attempts have been unsuccessful. In fact, the magnitude of the theoretical estimates of vacuum energy based on the quantum mechanics of matter and the value required to account for the acceleration of the expansion of the universe differ by an incredible factor of at least 10120 (which follows a 1 by 120 zeros ) Several other theories have been suggested, but the conclusion is that while there is compelling evidence that dark energy exists, we do not yet know the source of that energy.
Whatever the dark energy, we have to keep in mind that the discovery that the rate of expansion has not been constant since the beginning of the universe makes it difficult to calculate the age of the universe. Interestingly, the acceleration doesn’t seem to have started with the Big Bang: in the first billion years after the Big Bang, when the galaxies were together, gravity was strong enough to slow the expansion. As galaxies moves farther apart, the weaker the effect of gravity became. Several billion years after the Big Bang, dark energy took over and expansion began to accelerate.
The delay causes the age of the universe, estimated by the simple relationship T0 = 1 / H, to appear older than it actually is, while the acceleration makes it appear younger. Answer for an age very close to T0 = 1 / H. The current best estimate is that the universe is 13.8 billion years old with an uncertainty of only about 100 million years. We now know that Hubble’s constant changes over time. However, it is constant at all times throughout the universe. When we say the Hubble constant is approximately 70 kilometers / second / million parsecs, we mean that this is the value of the Hubble constant at the present time.
Comparing Ages
We now have an estimate of the age of the universe from its expansion. Does this estimate agree with other observations? For example, are the oldest stars or other astronomical objects less than 13.8 billion years old? After all, the universe must be at least the same age. In our galaxy and others, the oldest stars are found in globular clusters, which can be dated using the models of stellar evolution.
The accuracy of the age estimates of globular clusters has improved significantly in recent years for two reasons: First, the models of the interior of globular clusters have been improved, largely thanks to better information on how atoms absorb radiation as they move away from the center of a globular cluster. Second, observations from satellites have improved the accuracy of our range measurements to these clusters. The bottom line is that the oldest stars formed around 12-13 billion years ago.
This age estimate was recently confirmed by examining the uranium spectrum in stars. The uranium-238 isotope is radioactive and decays (turns into a different element) over time. We know how much uranium is generally produced compared to other elements. Suppose we measure the amount of uranium relative to the non-radioactive elements in a very old star and in our own sun and compare the abundances. With this data we can estimate how long the uranium in the very old star has decayed, because we know from our own sun how much uranium has decayed in 4.5 billion years.
The uranium line is very faint and difficult to distinguish even on the Sun, but it has now been measured with the European Very Large Telescope on an extremely old star. If one compares the abundance with that of the solar system whose age we know, astronomers estimate the age of the star at 12.5 billion years, with an uncertainty of about 3 billion years. Although the uncertainty is great, this work is an important confirmation of the age estimated from studies of the stars in the globular cluster. Note that the estimated age of uranium is completely independent; It does not depend on the measurement of distances or models of the stars’ interiors.
the stars in the globular cluster probably did not form until the universe was expanding for at least a few hundred million years. As a result, their age coincides with the age of 13.8 billion years estimated from the rate of expansion.
