The Big Bang model must explain the present attributes of the universe, but it also needs to account for the past and it must make predictions about the future, even if we're not here to observe whether that actually happens. Since 100 million years or so after the Big Bang, the universe has entered the era of galaxies. It's become cold, huge, and very low density. Almost all the matter and a lot of the dark matter has clumped into physical objects and scales the size of galaxies, about a million light years across. The spaces between galaxies are not devoid of interest or of substance. There's a lot of dark matter in the spaces between galaxies and they're also must be the pervasive dark energy that's leading to the current accelerating expansion. Recall that the rapid early expansion means that there are regions of space that we haven't seen yet. In the evolving universe, the deceleration was followed by acceleration. That creates some interesting situations. It's possible for example, that there are regions of space that were ripped from our view by the faster than light early expansion that subsequently came into our view or entered our horizon because of deceleration. Then in the future or soon, maybe ripped from our view again by acceleration due to dark energy, an extraordinary situation. Remember that the Big Bang model does not talk about an edge to the universe. Operationally, the edge of our vision is determined by time and the time since the Big Bang, not by any limit to space. In the Big Bang model, space itself could be infinite. We currently have almost no information on dark energy and whether it varies over time and space. If dark energy does decay with time, the universe may not accelerate forever. But if it does, the galaxies we currently see with our telescopes will gradually be ripped from view by the accelerating expansion. Until all but the nearest galaxies, which probably will have merged with the Milky Way as Andromeda plans to in a couple of billion years, will have been wrenched from our view. Extragalactic astronomy will be over. We can understand the behavior of the universe in terms of a space-time diagram, often used in Physics. Time is plotted on the vertical axis on space compressed to one dimension on the horizontal axis. Speed therefore is the slope of a line in this diagram. By convention, the speed of light, the fastest way a signal can propagate is represented as a 45-degree sloping line. In a space-time diagram like this, the realm that's available to us is called our light cone. It's defined by the 45-degree angle moving upwards as time passes forward. There are regions beyond the sloping lines, which are further from us than light can propagate, which are invisible to us. Regions is therefore the weak-end influence. Similarly going back in time, the reverse of our light cone represent regions that can influence us or have influenced us in the past. In addition, there are regions outside our past light cone that can never have influenced us. The light cone is therefore the boundary of the places in the universe in one dimension that can communicate with us or that we can communicate with. Similarly, we can think of the universe in terms of light cones. Imagine a universe that was infinite and created instantaneously. As light reached us from more distant regions, the visible universe would grow one light day larger everyday, one light year larger every year, and we'd see larger and larger volumes as time went by. The situation is more complex than that in several stages. First, we have a Hubble expansion. So let's just imagine it as uniform expansion, a constant slope in a Hubble diagram. This means that galaxies have been moving apart at a regular pace for the entire history of the universe. So what we can see is therefore bounded by the age of the universe. We could only see in 13.7 billion light years in any direction in a universe that's 13.7 billion years old. But as I've already mentioned, the size of the observable universe in any direction is more than 13.7 billion light years. It's close to 46 billion light years, three times larger. A 27 or almost 30 times larger volume. The reason for this is of course that the universe was expanding much faster than light speed at the beginning. The superluminal expansion creates a distance to the edge of the observable universe, substantially larger than a naive calculation of 13.7 billion light years. The distance at which a galaxy is moving away from us at the speed of light define something called the Hubble sphere or the Hubble volume. But there are clearly regions of space in the Big Bang model beyond this that we cannot yet see. We may be able to see these in the future. But of course the universe has been decelerating and is now accelerating, which means regions may remain forever hidden from view and some of the regions we currently see may in the future be unobservable. It's very complex due to the varying expansion rate over cosmic time. The brake and the accelerator on the universe are dark matter and dark energy. Dark matter dominating early on, the first two-thirds of the universe's life and dark energy in the last third. It's extraordinary that dark forces govern the behavior of the universe. In traditional cosmology before dark energy was known, it was assumed that the only agent that could alter the expansion rate was mass, dark matter, or visible matter. Therefore, the universe might be closed or open. Closed implying a re-collapse in the distant future if the mass in the total universe was sufficient to overcome the cosmic expansion and open or infinitely expanding if that mass was insufficient to overcome the cosmic expansion. As much as we understand dark energy, which isn't very much, we're confident now that the universe will expand forever at an accelerating rate, with more and more objects being ripped from the view of our telescopes, because if they're accelerating expansion rate away from us. Within this decelerating and then accelerating universe, gravity sculpted all the objects we see. A 100 billion galaxies, their constituent stars and planets, and biology. How do cosmologists measure the age of the universe? Basically, the age is determined from the expanding universe and Big Bang model. In a simple model of continuous expansion, a Hubble constant has the dimensions of inverse time and its reciprocal is a rough guess at the age of the universe. That's essentially an extrapolation back of linear expansion to a point. Taking the reciprocal of a Hubble constant slightly more than 70 kilometers per second per megaparsec, gets an age of just under 14 billion years. That's close to the correct number. But it's more complicated than a simple linear extrapolation because the universe has been decelerating and then accelerating. The exact age calculated in the cosmological model is 13.7 billion years. It's actually accurate to a few percent or a few 100 million years. It will be nice to have an independent corroboration that the age of the universe is a number like that. In fact there are two methods that can be used. Both depend on our knowledge of stellar astronomy and therefore are independent of the cosmological world model. White dwarfs are among the oldest stars we can see in the universe. Some of them will cool essentially infinitely, radiating like black bodies and cooling embers forever into space. The cooling rate or cooling curve of a white dwarf is well determined by astrophysics. So if we find and measure the properties of the oldest white dwarfs in the universe, they give us an independent estimate of the age of the universe. Similarly, we can do this for the oldest stars in globular clusters, which were the first stellar conglomeration formed in the oldest galaxies like elliptical galaxies. Both methods give ages of about 11 to 13 billion years. The ages are quite inaccurate perhaps only 10 or 15 percent, but it's reassuring that they agree with the age from the Big Bang model. A second, also completely independent and different method, involves age dating using radioactive isotopes. As with radiocarbon dating, we can use a radioactive isotope with a half life of thousands of years to measure the age of objects about that age. Using isotopes of uranium and plutonium that have half lifes of billions of years, we can measure the age of a multi-billion year old universe. This method is quite uncertain. But once again, it gives an age roughly 10 to 15 billion years. It's very important these two completely independent methods corroborate the age from the Big Bang model. This crosscheck need not have been the case. If there were objects in the universe older than the Big Bang model predicted, it would have been a fatal blow to the Big Bang model. As it is, the Big Bang model passes this test. Although there's exotic physics in the early universe, for most of its history, it's been doing a steady expansion, becoming thinner, less dense and cooler as galaxies and stars continue to form within it. The peak of galaxy formation lies long in the past. Star formation also is declining with time. The expansion rate decelerated initially and then has been accelerating due to the dominance of dark energy leading to a prediction of an infinitely expanding universe. The early expansion rate was fast enough to guarantee that there are regions of space that we've never seen and some that we may never see.
