The universe is a big place and it's getting bigger! Like everything else, we expect the universe to have a size, a shape, a beginning and an end. The study of the nature, origin, evolution and fate of the universe is called cosmology. It's an exciting and difficult area of science involving math, physics and astronomy. Tonight I will try to explain our current understanding of the "cosmos" as understood by cosmologists.
Cosmology is a BIG subject and galaxies are the fundamental "unit" of cosmology. You know, from the previous lesson, that all the galaxies, outside our Local Group, are red shifted regardless of the direction we look. From this we conclude that the universe is an expanding sphere.
Will the universe continue to expand?
Good question.
One of the big arguments in 20th century cosmology has been whether
the universe will continue to expand or whether it will eventually
collapse under its own gravity. The answer depends upon how much
mass there is in the universe. It's all got to do with mass, gravity
and velocity. If you throw a ball into the air the Earth's gravity
will pull the object back down. However, if you move that ball
very fast (over 11.2 kilometers per second) it will escape the
Earth's gravitational tug and drift into space, never to return
to Earth. Similar physics applies to our expanding universe. To predict the fate of the universe first find the velocity and positions of all galaxies. We already know those values using Doppler shifts and Hubble's constant. Next figure out how much mass there is in the universe, among all the galaxies, and use that value to estimate the gravitational attractions. So the question is, "Does the universe have enough mass to stop the galaxies from receding forever?"
Calculations can be done, using what we know about the velocity and position of galaxies, to determine the critical density - the mass density of the universe needed to produce enough gravitational force to counter the expansion. This critical density has been estimated to be 1.9 x 10-29H2 grams per cubic centimeter. The "H" is Hubble's constant (it's NOT meant to mean hydrogen) and you will recall that the value of H has not been pegged down completely. (And some astronomers will argue that H is way off because it uses red shifts.) If we run through all the math and use a value of H that most astronomers accept we find that the critical density is equivalent to approximately 10 protons per cubic meter. Astronomers like to use simple numbers when presenting their arguments so they use Omegam to represent the ratio of the true density of the universe to the critical density.
(Omegam = density of the universe / critical density)
If Omegam exactly equals 1, the density of the universe equals the critical density and the universe has exactly enough mass to stop the expansion. It means the universe would stop at some predefined size (predefined
by its mass) and sit at that size forever. It is the equivalent to throwing a ball away from Earth with exactly the velocity needed for it to never return to Earth but never go further than a certain distance.
However, this example with the ball shouldn't be taken too strictly. First, it is the mass of the ball NOT its velocity we should be considering. Second, you might think from my example, that this would not be too difficult a feat because we put things into orbit all the time. However, an orbit is NOT what we are talking about here. Instead we are assuming that the mass and velocity of the ball are such that the ball would end up motionless in space! That is next to impossible. The "exactness" of the values would be impossible to meet in practice.
Indeed, it is this "exactness" that has always bugged me (and others) because it is so unlikely to have been so correct.
If Omegam is greater than 1, the density of the universe is greater than the critical density and the universe will eventually slow down, stop expanding, pull back, and eventually fall back from where it came. This is equivalent to throwing the ball at less than its escape velocity such that it falls back to Earth (but, again, we should be thinking about varying the mass not the speed). In this scenario, the universe ends up in a "Big Crunch" with all mass being crushed into the ultimate black hole!
Until recently, many astronomers (religiously?) held to this view.
If Omegam is less than 1, the density of the universe is less than the critical density and the universe will expand forever. This is equivalent to throwing the ball faster than escape velocity (but, again, we should be thinking about varying the mass not the speed).
OK, what is the density of the universe?
Determining the mass, and from that the density, of the universe is not an easy chore but careful observations and calculations allow us to get a good estimate.
First astronomers calculate the mass of all the stars and galaxies they can see. Actually, they observe a small sample of the sky and use statistics to calculate the rest. That gives a value for the mass of all the "bright" objects in the universe. Then they must take into account the "dark mass" - the material that makes up planets and nebula and other things that do not shine (by fusion). These masses are determined by observing how fast galaxies rotate. The physics and math behind this follows the same mechanics as determined by Issac Newton three centuries ago so it is fairly well understood. Again, they observe only a small sample of the sky and use statistics to calculate the rest. When they add up the masses of all the stars and dark mass it comes to only 10% of the mass needed for the universe to "fall" in on itself. (I hinted at this in our last lesson when I told you the universe had an average of one proton per cubic meter.)
So, Omegam < 1. Therefore,
the universe will continue to expand because it does not have
enough mass to halt its expansion.
Some astronomers insist that there is enough mass in the universe to cause the collapse. They argue that we just haven't found it all yet. Some theoretical physicists suggest a totally different kind of matter, which they call "dark matter", makes up the missing mass. Don't get confused - this "dark matter" is not the "normal" dark mass we associate with non-fusing masses, like planets and nebula. What these folks are proposing is a weird, new kind of matter. There is no proof that this dark matter even exists! Faith in this hidden mass or matter is based upon a "hunch" that the universe MUST collapse again. There is no evidence to support their opinion. Some of these folks also resort to some way-out thinking involving radical concepts like "vacuum energy", "anitigravity" and similar areas of science that are, at best, unusual concepts with modest evidence that they might be useful in our understanding of the mass of the universe. I think it's interesting to note that when it comes to the big issues, like the end of the universe, some people (some of whom are devoted atheists) can display an unscientific "faith" in order to justify their view. This is purely a matter (yes that's a pun ![]() |
The current evidence suggests that the universe will expand forever and eventually die as it runs out of atoms capable of nuclear fusion. The skies will go dim as nothing but blackholes dominate the universe. Eventually even these blackholes will evaporate (probably - it's getting into some weirdness again). But don't let that worry you because it won't happen for a very long time. Relatively speaking, the universe is still quite young.
OK, so the universe will continue to expand forever until the day it dies in the far off future. But how old is the universe and where did it come from?
An excellent question.
All civilizations have their creation myths about their origins.
Science tries to address those questions using observations and
experimentation but there was no one around when the universe
was created and you cannot make a new universe in the lab, so
we have to depend upon analysis of the current data. This is
a difficult area to deal with in detail but I hope to convince
you that cosmology has a good idea about the origin of the universe.
Our scientific understanding about the beginning of the universe started with a man named George Lemaitre. George was one of those people fascinated with "deep" problems. He started as an engineer but at 21 he decided to study math and physics instead. At the age of 22 he became a Roman Catholic priest. He was awarded a PhD from the Massachusetts Institute of Technology in 1927 and moved to Belgium where he became a professor of astrophysics at the University of Louvain. That year, at the age of 31, he published his ideas concerning the origin of the universe. He reasoned that the expansion of the universe, observed by Slipher in 1920 and developed by Hubble, meant that in the past the universe had been smaller and denser. Lemaitre imagined that the expansion of the universe would look like a contraction if run in reverse. This suggested to him that the universe was born in a dense explosion at a specific time in the distant past. He teamed up with another astronomer named George Gamow and together the two Georges developed this idea. They suggested that all the matter in the universe was at one time concentrated in one small dense mass called the "primeval atom" and that it exploded (for some reason) sending all its matter outward in all directions. This work of the two Georges was originally ignored until the famous astrophysicist Arthur Eddington called attention to it. Gamow went on to do other great work in astronomy. Lemaitre's contribution was also recognized and at the time of his death (1966) he was president of the Pontifical Academy of Sciences in Rome.
The idea generated by the two Georges has come to be known as the "Big Bang Theory" and it is now generally accepted as the best explanation that science can come up with for the origin of the universe. Astronomers and physicists have worked on the idea and added new information about this amazing event. They have produced models that simulate the birth of the universe or at least what happened soon after its birth.
It is convenient to divide the history of the universe into three very unequal time periods.
Of course, the most important part of the story is the quantum cosmology part because it's the secret to the origin of the universe. I won't teach this part to you because I am not qualified to do so, I don't understand it anyway, and it is so speculative that I couldn't recite the story to you while keeping a straight face . Let's just accept that at the very beginning of the Big Bang, during the Planck epoch, there was a tremendous release of energy that started the whole universe.
The energy at this time would have given the newly born universe a temperature of 1033 (a one followed by thirty-three zeros) degrees Kelvin. That's hotter than anything I can imagine and hotter than anything created by man. At those energies matter, as we know it, cannot exist and even the very forces of nature (like gravity, electromagnetism and the atomic forces) would not have been around!
This ball of energy expanded rapidly. Some say it did so faster than the speed of light ! (Some say Elvis is still alive.
) Regardless, as this ball of energy expanded it cooled, behaving like a gas, and as it cooled it entered into the area we like to think of as particle cosmology. First came gravity, then the weak nuclear force (involved in the interactions of things like electrons) and electromagnetism (light) and finally the strong nuclear force appeared (which make the atomic nucleus so stable). Physicists can create energies that will reverse this pattern as far back as the weak nuclear force and watch as the other energies come out of the mix but they cannot go back as far as the formation of gravity. Also, we do not have a proper theory about quantum gravity upon which to build a good story so, frankly, we don't know what happened during the time of quantum cosmology or the earliest stage of particle cosmology. However, we do know what happened in that tiny fraction of a second that makes up the later part of the particle cosmology time.
With the strong force now in the universe, sub-atomic particles (called quarks) could coalesce into stable nucleons - protons and neutrons. Matter, as we know it, was created around 0.000001 second after the Big Bang.
The story is all down hill from here (and less likely to be wrong)!
As the universe continued to expand it continued to cool and about one second after the Big bang some of the nucleons (protons and neutrons) condensed into the light elements; hydrogen (H), deuterium (D - an isotope of hydrogen so it would be correct to abbreviate it as 2H), helium-3 (3He) and helium-4 (4He). Theory predicts that these should be produced in specific ratios. Hydrogen should make up the bulk of the new materials, helium-4 (4He) should account for about a quarter and smaller amounts of the others. These ratios agree with what we observe to be the main constituents of the universe - when corrected for the nuclear synthesis from stars which occurred much later. By the way, it was still too hot for electrons to take up stable orbitals around the nucleons so the universe was still in an ionized state. It was a plasma universe!
A couple minutes after the Big Bang "gravitational instability" started to occur which means that, instead of having a nice clean distribution of matter evenly spaced in all directions, we ended up with some places having higher densities than others. And it's a good thing too! Without this instability, matter would have just been scattered in such a way that masses could not have been attracted to each other in a significant way. In other words, materials would not have accreted together. Thanks to this instability, material was able to collect to form stars and galaxies.
When did real atoms, with electrons, form?
About 100,000 years after the Big Bang the temperature of the universe would have cooled to about 3000 Kelvin. That's cool enough for hydrogen atoms to hook up with electrons and proper atoms can form. This caused an important effect which is difficult to explain in detail because it has to do with some advanced physics. When the universe was still ionized the energy from the Big Bang was bounced around in a cloud of ionized materials (called a plasma and behaving like an opaque gas). Remember how gamma rays are bounced around in the center of the Sun for millions of years before they reach the surface? Well, the idea is similar, but not exactly the same. Anyway, once atoms had formed, the background energy leftover from the Big Bang, was able to propagate freely through the universe (like a photon finally making its way through the photosphere). The wavelength of that energy is calculated to be stretched by the expanding universe and interactions such that by now the background radiation would have a temperature of only three degrees Kelvin.
So, among the equations and explanations was the idea that the Big Bang should have left behind a signal - a "noise". Specifically, the energy of the Big Bang should have "cooled" and dissipated to give a low level "glow" of energy throughout the entire universe called the "background radiation". In 1965 two scientists, Robert Wilson and Arno Penzias working with a radio telescope at Bell Laboratories, detected this weak background radiation. [If you remember that heat is in the infrared you may also recall that below the infrared is a very low form of energy that we detect as radio waves. The background radiation must be very, very low energy and, therefore, in the radio part of the spectrum.] They found it was coming from all directions. The entire universe is full of this low level "noise" leftover from the Big Bang explosion. There's no other explanation. The noise was exactly the kind expected from the Big Bang. Wilson and Penzias eventually won a Nobel Prize for their discovery and are credited as the guys who proved the Big Bang.
However, some astrophysicists complained that the background radiation was too smooth, too consistent. They argued that the Big Bang should leave a spotty representation of the universe as the materials in it cooled and formed matter. They expected some variation in the background radiation. They admitted, however, that the variation would be hard to measure. In 1993 a satellite named COBE (Cosmic Background Explorer) found those tiny variations. COBE mapped the variation in the background radiation to produce a map of the "Ripples of the Big Bang".
OK, so once matter started to behave normally it formed stars and they made the other elements.
Right. Stars produced heavier elements and flung them into the universe when they went nova. Other stars formed and so did planets. On at least one of those planets life evolved and intelligent beings eventually evolved too. They looked at the sky and wondered how it all began.
We live in a unique time in our history. At no other time have we been able to ask questions about the universe that can be answered by scientific instruments, observation and evidence.
So that's it? We now know it all?
No. We know a lot but not all of it. And, because science is based upon observations and experimentation, our opinions of the origin and fate of the universe might change one day. But each day the accumulating evidence is more and more convincing that the universe started in a Big Bang and the universe will end as a forever expanding starless sphere.
However, the Standard Cosmology Model, which I have struggled to teach you here, is NOT without its problems. Some astronomers think the background radiation is a little too smoothly distributed. Where and how did these all-important gravitational instabilities come from? Without them, we would not be here. There are many unanswered questions arising from quantum mechanics and relativity. Our greatest astronomical instrument, the Hubble Space Telescope, has provided us with a wealth of data but some of that data is hard to interpret. Some astronomers think the data tells us that some stars and quasars are actually older than 12 billion years - but how can that be? Indeed, some astrophysicists argue that 12 billion years is not enough time for certain characteristics of the universe is to have developed at all!
There have been other scientific theories about the origin and fate of the universe. A few decades ago a very popular origin theory was that the universe is still being created! This "Steady-State Theory" maintains that, as the universe expands, new matter is created between the galaxies. Let's face it - if you can accept the idea that matter was created at one point at one time (the Big Bang) it is hardly a stretch of the imagination to think that it could still be created in the far off distances between galaxies. It turns out that you don't need to create much new matter to keep the Steady-State. The amount of newly created matter would be barely noticeable and it would be happening far away from us in intergalactic space so it would be hard to see. A key feature of the Steady-State Theory is that the distribution of galaxies should remain the same. Indeed, the universe should be very much the same everywhere because the Steady-State theory says creation has been and always will be constantly occurring.
It's this requirement in the Steady-State theory for an unchanging universe that is so appealing and exciting. It is also its downfall because we know that the universe is "aging". It isn't the same as it was many billions of years ago. Naturally, we cannot build a time machine to go back into the past but we can use powerful telescopes to look at the universe as it was billions of years ago. Remember, the further away a galaxy, the longer its light has traveled to get here so its light gives us an image of that galaxy billions of years ago. If the Steady-State theory is correct, galaxies near the edge of the universe, over ten billion light-years away, should look the same as our own neighbors. (Not counting, of course, the red shifting.) When powerful telescopes look out into distant space and look back into the earliest times of the universe, astronomers see that the early universe was not the same as it is today. Conditions were different and there were some strange things in the distant parts (distant past) of the universe like nothing in our neighborhood today.
Like what?
Like quasars. It appears that quasars are a property of the distant past. There are none in our neighborhood and, therefore, none in our recent history (measured in billions of years). Therefore, the universe of the distant past was different from our current universe and that means the Steady-State theory is
not correct.
[Also, the background radiation discovered by Wilson and Penzias
as well as its "ripples" discovered by COBE cannot be
explained by the Steady-State theory. The Big Bang Theory not
only explains them but expects them!]
But who knows. Maybe quasars are leading us astray. Maybe they aren't so far away in space and time.
That's the amazing thing about astronomy and cosmology. New data
is constantly coming in. We have to learn to explain the universe
as we find it, not simply as we would like it to be. Years ago,
when I started studying astronomy, there was a popular theory that
our expanding universe would one day in the far future slow
down and collapse upon itself in the opposite of the Big Bang,
called the Big Crunch. We now know that there is not enough matter
in the universe to do that. (Although some folks might disagree. ) We didn't know that just a few years ago! Many (most?) astronomers and cosmologists were "fans"
of the Big Crunch idea. They liked it, probably because it implied
that after the Big Crunch there would be another Big Bang and
a new universe would form and go on to evolve again. Then it would
slow down, contract, Big Crunch, Big Bang and so on, in an infinite
series of creations and destructions.
This theory is called the Oscillating Universe Theory and it's very "attractive". It appeals to our feelings. It allows for a new universe to be created, again and again. It smacks of reincarnation and rebirth. It seems like a good idea. A good idea is enough for philosophers and theologians but scientists know that data, evidence and experimentation are the only way to come to a scientific conclusion. The recent data say there isn't enough mass in the universe to cause it to contract so the Oscillating Universe is nothing more than a dream. At least until new data prove other wise.
Maybe one day a bright, careful
astronomer will find the missing mass, saving the Oscillating
Universe Theory and changing, once again, the way we look at the
universe.
Maybe it will be you!
There is a big universe out there and I hope these lessons have helped you to understand at least a part of it. With your education you should now be able to identify and explain a great deal of the universe. Enjoy it!
Wishing you "Clear Skies".
Jamie (Dr Love)