COSMOLOGY
Reliable astronomical records are available for the last 100 years or so. While this may look good enough for a detailed study and research, this is just a minute time interval by astronomical scales. Remember, our universe is 13.8 billion years old! And this is why these astronomical scales are typically in the order of 10 billion years. To study the evolution of the universe, astronomers rely on observations of distant objects. For example, the Andromeda Galaxy (M31) is about 2 million light years away. This means that our photographs of M31 show the galaxy as it was 2 million years ago! Why? Simply because we are measuring in terms of the speed of light, i.e. light years. So the photons which got ejected from the Andromeda galaxy took 2 million years to reach us and hence we see that old picture only now! Kind of a way to look into the past of the universe.
The Virgo cluster of galaxies is about 50 million light years away, and the information presently received by our telescopes describe the state of the cluster as it was 50 million years ago. Information received from galaxies that are billions of light years away pertains to the state of these objects as they were billions of years ago, when they were much younger, possibly just forming. Isn't it interesting how we can actually take a sneak into the past? These information are vital in reconstructing the evolutionary stages of the galaxies, and the universe as a whole. As the development of testable hypothesis is limited by our present instrumental capabilities, the proposed theories by numerous astrophysicists are speculative.
HUBBLE'S LAW
Edwin Hubble (20 Nov 1889 - 28 Sept 1953) was an astounding American astronomer who is most famous for proving that the universe extends far beyond our Milky way galaxy and for discovering that our universe is constantly expanding. He was the one who opened the gates of cosmos (short for cosmology)! He determined that there is a relation between the distance of various galaxies and their radial velocity, derived from the doppler shift of absorption lines in the spectrum of galaxies. Okay I know a lot of technical words thrown right at your face, but lets analyse it and break it down to simple terms. First up, radial velocity is the perpendicular component of any velocity vector. Say a galaxy is rotating with some velocity, then if we resolve its velocity vector into two directions such that one is tangential to the curve of rotation and the other is perpendicular to it, then that perpendicular component is called the radial velocity.
The Doppler shift of absorption lines in a galaxy's spectrum is the stretching or compressing of its light wavelengths due to its motion relative to Earth. When moving away, wavelengths lengthen (redshift), when approaching, they shorten (blueshift). Astronomers use this shift to measure a galaxy's velocity. Now let's come back to Hubble's observation. He found that all spectral lines appear red-shifted for all galaxies. This result proved that all galaxies are moving away from us, that is the UNIVERSE IS EXPANDING!
The amount of red shift in the absorption is proportional to the distance of the galaxy. This is also known as Hubble's Law, mathematically given as:
v ∝ d
v = Hd
where v is the radial recession velocity, d is the distance between galaxies and H is the Hubble constant. Current best estimates of H are around 65 km/s/Mpc (megaparsec). The only galaxies that deviate from Hubble's Law are those so close that gravity dominates their behaviour.
HUBBLE'S LAW AND THE EXPANSION OF THE UNIVERSE
If the galaxies are just moving randomly through space, some near ones would move rapidly and some distant ones would move slowly. About half as many would approach as recede. The fact that all the galaxies are moving away from us doesn't mean that we are in a special place in the universe. Hubble's Law could be used at any location in the universe, with the same result. This means that space itself is expanding, because the more distant the galaxies are, the faster they are moving away. A 2 dimensional analog is the surface of the expanding balloon. If you blow up a balloon, and mark little dots on it to indicate galaxies, then continue to blow up, you will find that the distance between the dots that were most separated increased more than the distance between the dots that were close together. In the same way, more distant galaxies move away faster, and so we know that the space in between grew. Fortunately, there is more evidence than simply the Hubble's Law, and this evidence has convinced astronomers that the universe is indeed expanding.
HUBBLE'S LAW AND THE AGE OF THE UNIVERSE
According to Hubble's Law, the distance d divided by the recession velocity (recession because its velocity of separation) v is equal to the constant 1/H, the same for all galaxies. The ratio d/v gives the time it took for the Universe to expand to the present state. This implies that 1/H is the age of the Universe. Using H = 65 km/s/Mpc, and values from the sources, we can estimate the age of the Universe to be about 15 billion years.
An important assumption in this estimate is that the rate of expansion is constant. The gravitational interaction between galaxies would tend to slow down the expansion.
The speeds in the past were slower, so the expansion has accelerated . The expansion age of the universe is often referred to as the Hubble Time, and depends on the value of Hubble's constant, which may include significant uncertainty. The uncertainties lead to some paradoxical results. For example, some globular clusters and white dwarfs seem to be older than 15 billion years, which is inconsistent with the Hubble time, like how is that possible? Well, the error may result either from stellar evolution calculations or in the determination of distance (d) and velocity (v), or simply in the assumptions itself.
So back then the astrophysicists thought that "The universe is probably not younger than 15 billion years old. Though it may be older, it's probably not very much older :)". But let me also tell you, the universe is not exactly 15 billion years old. According to the standard cosmological models and measurements and the latest space studies, we have found that the age of the universe is close to 13.82 billion years old (According to the European Space Agency). The estimate is primarily derived by looking at the oldest light in the universe - the Cosmic Microwave Background (CMB) and applying our understanding of the universe's expansion rate.
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