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Cosmology

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Cosmology

As we have learned, Edwin Hubble showed that the farther away a galaxy is, the faster away from the Milky Way it is moving. This gives the appearance that the Milky Way is at the center of the universe, and all galaxies are moving away from us, possibly due to some large explosion, The Big Bang. The Hubble's Law can be applied to any observer in any galaxy. No matter where you are, an expanding universe will give the same appearance. The expansion of the universe is not like the explosion of a bomb sending fragments in all directions. Space itself is expanding.
We can detect photons that appear to have moved at different speeds through space. However, as the speed of light is constant, it is space that is moving relative to the photon. The galaxies are fixed relative to space, but space itself is moving.
We have seen no 'edge' to the universe and there are an equal number of galaxies in every direction. Also, galaxies can move relative to space, but at times gravity can accelerate one galaxy toward another faster than space expands.
When light waves travel through space, they are stretched by expansion and this increases the wave's wavelength, making it appear redder.
Using the Hubble Law, we can estimate the age of the universe. At some point in the distant past, matter in the universe must have been densely packed. From this point, the universe would have expanded at some high speed to become today's universe. Assuming a constant expansion over time, we find that the age of the universe is around 14 billion years.
Over very large distances, galaxies in the universe are more or less uniformly distributed. If there are galaxies in every direction we should see a star in any direction we look and if there is an edge to the universe, we should be able to see our way out.
In a sense, there is an edge to the universe, an edge in time. Light travels at a finite speed. The size of the visible universe is the distance light can travel in the age of the universe. Galaxies exist at greater distances, but light from them has not reached us yet. The edge is called the cosmic horizon. If we wait long enough, the night sky might become bright.
Every time we look at the night sky, we are looking back in time. It takes more than four years for light from the closest star to arrive. It takes billions of years for light from the most distant galaxies to arrive.

Can we see light from the Big Bang? Almost. Minutes after the Big Bang, the universe was opaque. High temperatures kept all matter ionized and photons could only travel a short distance before being absorbed. After about 400,000 years, the universe cooled enough for electrons and ions to recombine, allowing light to pass and the universe became transparent.
So what should light from 400,000 years after the Big Bang look like? It should have a spectrum that corresponds to the temperature of the universe at that time of about 3000 K. Expansion of space will stretch this light. The universe has expanded by a factor of 1000 since that time, so the wavelength will have stretched by the same amount. Thus the present-day spectrum should correspond to a temperature of about 3K. This is what we indeed observe today as we can measure the cosmic background temperature of about 2.7 K in all directions in the sky.
This light from the early universe has been found, and is called the Cosmic Microwave Background. The universe became darker and darker as the cosmic microwave background became more and more redshifted. Using the Hubble Space Telescope, we can image galaxies as they appeared 13 billion years ago. These galaxies must have formed "shortly" after the Big Bang and the galaxies must have been able to form relatively quickly.
As the universe expands, the distances between the galaxies increase. If all points of the universe were to reverse in the opposite direction, eventually everything would return back to the “starting point”.
Recent measurements show that space is nearly flat. Immediately after the Big Bang, only protons and electrons existed. Shortly after the Big Bang, temperature and density was high enough for deuterium to form by fusion. After about 100 seconds, temperature cooled enough so that deuterium could fuse into helium nuclei. The temperature continued to cool, and fusion stopped after a few minutes. Big Bang theory predicts that around 24 percent of the matter in the early universe was helium, which matches what we observe.
In the first half of the early universe, matter did not really exist; rather, everything was radiation or energy. This time period is called the early universe, which then underwent a period of very rapid expansion.

The fate of the universe is ultimately controlled by its total amount of energy: positive energy of expansion, negative gravitational energy that can slow the expansion, and the binding energy. If the total energy is positive or zero, the expansion continues forever. However, if the total energy is negative, the expansion will stop and the universe will contract and eventually collapse. If we can measure the density of the universe, we can predict how much gravitational energy the universe has, and therefore, whether it will collapse or keep expanding.
For a while, things will be pretty much as they are now. The Sun will orbit the center of the Milky Way. Andromeda Galaxy will appear to increase in size as it draws closer. During the collision between the Milky Way and Andromeda, our solar system will probably be flung into intergalactic space. After our Sun dies, the planets will be stripped away, including Earth. Over the next 10 years, galaxies in the Local Group will merge.
Star formation will slow as gas is slowly locked up in stellar cores. Eventually, the only objects left are cooling stellar corpses and radiation. Expansion of the universe will continue to accelerate, as galaxies become widely separated. Eventually, everything will be locked up in black holes. Even these will decay with time.

Life formed on Earth relatively soon after the planet's formation. However, about 75 percent of the Earth's history, only algae and single-celled life forms existed. Slowly, more complex forms of life developed. By 250 million years ago, dinosaurs and early mammals had evolved. Hominids, our distant ancestors, developed 5.5 million years ago and Homo sapiens evolved only 500,000 years ago.
Life tends to draw on the substances that are most abundant: carbon, nitrogen, oxygen, and hydrogen. Amino acids are organic molecules formed from these elements. Amino acids form proteins, which provide structure and energy to cells.
Amino acids were formed due to complex chemical reactions in the atmosphere and surfaces of Earth. This was shown by the Miller-Urey experiment attempted to duplicate the environment of the early Earth.
The first cellular life on Earth had no internal structures, making them similar to simplest bacteria, prokaryotes. These bacteria reproduced rapidly, and were the only form of life until two billion years ago.
Early atmosphere was very different than it is today. It contained less oxygen, which may have been deadly for the development of life. Today's prokaryotes are found in low-oxygen environments. Later cyanobacteria resulted in an oxygen-enhanced atmosphere.
Eukaryotes arose, blooming in the oxygen-rich atmosphere. Multi-celled organisms only began to appear about 600 million years ago. The greater complexity of multi-celled organisms would allow them to exploit resources unavailable to simpler organisms: they can move to better environments and be more adaptive.

It appears that Mars at some point in its history was very much wetter and warmer than it is today. Using modern methods and instruments scientists have been looking for life there. The Viking landers (1970's) tested for the presence of microbes, but returned inconclusive results. Nevertheless, the search for life on other planets is continuing.

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