of the Universe|
Universe Today :
The present-day Universe is a rich collection of galaxies of many types, clusters of galaxies, large scale structure and exotic phenomenon (e.g. Galactic black holes). The galaxies themselves contain stars of all sizes, luminosities and colors, as well as regions of gas and dust where new stars form. We suspect that many stars have planets, solar systems in their own right, possible harbors of life.
So what's going to happening in the future??
If the Universe is closed, then we might expect the arrow of time, as defined by entropy to reverse. There appears to be a natural connection between the expanding Universe and the fact that heat moves from hot areas (like stars) to cold areas (like outer space). So if the expansion of space were to reverse, then would entropy run the other way?
This kind of Universe has no real beginning or end, and is refered to as an oscillating Universe. Notice that it's impossible to determine which side you currently are on since time reverses and all appears normal to the observer.
The Fate of the Universe :
The past history of the Universe is one of an early, energetic time. As the Universe expanded and cooled, phenomenon became less violent and more stable.
This ruling law of Nature during the evolution of the Universe has been entropy, the fact that objects go from order to disorder. There are local regions of high order, such as our planet, but only at the cost of greater disorder somewhere nearby.
If the Universe is open or flat (as our current measurements and theories suggest) then the march of entropy will continue and the fate of our Universe is confined to the principle of heat death, the flow of energy from high regions to low regions.
With this principle in mind, we predict the future of the Universe will pass through four stages as it continues to expand.
Stellar Era :
The Stellar Era is the time we currently live in, where most of the energy of the Universe comes from thermonuclear fusion in the cores of stars. The lifetime of the era is set by the time it takes for the smallest, lowest mass stars to use up their hydrogen fuel.
The lower mass a star is, the cooler its core and the slower it burns its hydrogen fuel (also the dimmer the star is). The slower it burns its fuel, the longer it lives (where `live' is defined as still shining). The longest lifetime of stars less than 1/10 a solar mass (the mass of our Sun) is 1014 years.
New stars are produced from gas clouds in galaxies. However, 1014 years is more than a sufficiently long enough time for all the gas to be used up in the Universe. Once the gas clouds are gone, all the matter in the Universe is within stars.
Degenerate Era :
Once all the matter has been converted into stars, and the hydrogen fuel in the center of those stars has been exhausted, the Universe enters its second era, the Degenerate Era. The use of the word degenerate here is not a comment on the moral values of the Universe, rather degenerate is a physical word to describe the state of matter that has cooled to densities where all the electron shell orbits are filled and in their lowest states.
During this phase all stars are in the form of white or brown dwarfs, or neutron stars and black holes from previous explosions. White and brown dwarfs are degenerate in their matter, slowly cooling and turning into black dwarfs.
During this era, galaxies dissolve as stars go through two-body relaxation. Two-body relaxation is when two stars pass close to one another, one is kicked to high velocity and leaves the galaxy, the other is slowed down and mergers with the Galactic black hole in the center of the galaxy's core. In the end, the Universe becomes filled with free stars and giant black holes, leftover from the galaxy cores.
The Universe would evolve towards a vast soup of black dwarf stars except for process known as proton decay. The proton is one of the most stable elementary particles, yet even the proton decays into a positron and a meson on the order of once per 1032 years. Thus, the very protons that make up black dwarf stars and planets will decay and the stars and planets will dissolve into free leptons. This all takes about 1037 years.
Black Hole Era :
Once all the protons in the Universe have decayed into leptons, the only organized units are black holes. From Hawking radiation, we know that even black holes are unstable and evaporate into electrons and positrons.
This process is extremely slow, varying inversely as the mass of the black hole. For Galactic mass black holes the time to dissolve can last up to 10100 years. The result is a bunch of photons, slowly cooling in the expanding Universe.
Dark Era :
After all the black holes have evaporated, the Universe consists of an expanding sea of very long wavelength photons and neutrinos. This is a system of maximum disorder, no coherent structures or objects. No sources of energy, and no sinks as well. The rest of time is simply a continual lower of energy until the state of quantum vacuum is reached.
Fermi's Paradox (i.e. Where are They?):The story goes that, one day back on the 1940's, a group of atomic scientists, including the famous Enrico Fermi, were sitting around talking, when the subject turned to extraterrestrial life. Fermi is supposed to have then asked, "So? Where is everybody?" What he meant was: If there are all these billions of planets in the universe that are capable of supporting life, and millions of intelligent species out there, then how come none has visited earth? This has come to be known as The Fermi Paradox. Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire Galaxy. Within a few million years, every star system could be brought under the wing of empire. A few million years may sound long, but in fact it's quite short compared with the age of the Galaxy, which is roughly ten thousand million years. Colonization of the Milky Way should be a quick exercise.
So what Fermi immediately realized was that the aliens have had more than enough time to pepper the Galaxy with their presence. But looking around, he didn't see any clear indication that they're out and about. This prompted Fermi to ask what was (to him) an obvious question: "where is everybody?"Also, if one considers the amount of time the Galaxy has been around (over 10 billion years) and the speed of technological advancement in our own culture, then a more relevant point is where are all the super-advanced alien civilizations. Russian astrophysicist Nikolai Kardashev proposed a useful scheme to classify advanced civilizations, he argues that ET would posses one of three levels of technology. A Type I civilization is similar to our own, one that uses the energy resources of a planet. A Type II civilization would use the energy resources of a star, such as a Dyson sphere. A Type III civilization would employ the energy resources of an entire galaxy. A Type III civilization would be easy to detect, even at vast distances.
This sounds a bit silly at first. The fact that aliens don't seem to be walking our planet apparently implies that there are no extraterrestrial anywhere among the vast tracts of the Galaxy. Many researchers consider this to be a radical conclusion to draw from such a simple observation. Surely there is a straightforward explanation for what has become known as the Fermi Paradox. There must be some way to account for our apparent loneliness in a galaxy that we assume is filled with other clever beings.
Estimates for the number of civilizations in the Galaxy are made using the Drake equation:
In general, solutions to Fermi's paradox come down to either 1) life is difficult to start and evolve (either hard for the process or hard to find the right conditions) or 2) advanced civilzations destroy themselves on short timescales. In other words, this is an important problem to solve in the hope that it is 1 and not 2.
This course has been an exploration into modern cosmology and the search for the final laws of Nature (a theory of Everything) and the origin of Universe. Although there are many, many unsolved riddles to the Universe, the basic picture known as the Big Bang model is, at the very least, the foundation whose basic properties will always remain unchanged.
Although many of the concepts discussed in this course are strange, they are all based on rational scientific thought (the real world is stranger than anything you can imagine). A proper scientific model leaves less room for irrational beliefs. Understanding within the scientific method removes the blank areas on our maps, the place were we once drew monsters and golden cities. This knowledge dampens our fears like a candle in the dark.