Relativity & Cosmology

Light travels with a finite speed. What it means is that whatever we see is information from the past. When we see the Sun or a tree close to us, we are seeing an image of it from the past.
But the speed of light is enormously high. So people were not able to measure it until 1668 to 1676 in spite of many prominent physicists such as Isaac Beeckman in 1629 and astronomers such as Galileo in 1638, attempted to find it out.
It is Danish astronomer Ole Rømer who made the first successful measurement of it and published it. He did it when tracing the orbits of Io and other Galilean satellites of Jupiter. But he did not get the precise numerical value of the speed of light as he did not know the distance of the satellite from the Earth. But his peers , mainly Christiaan Huygens and Edmund Halley were able to find out the exact numerical value of it. Read more...➔

Albert Einstein published the theory of Special Relativity in 1905 in two papers successively and is one of the most influencial theories ever produced in the discipline of physics.
Special Relativity was propounded to deal with how speed affects the time, space , mass and energy. Classical mechanics (laws of motion proposed by Newton) explains very well about the scenarios or events where the speeds that we experience in everyday life are involved. Even we are launching the satellites into the space from Earth using the Newton's mechanics.
But if you have to deal with situations involving speeds closer to the speed of light, you need a new way to describe them. That is where the Special Relativity comes into picture. Read more...➔

Light experiences Doppler Effect when traveling through the space and this results in the red and blue shift of the light frequency. But what is Doppler Effect first of all ?
When the source of the sound moves towards you or else if you move towards the source of the sound , you will hear the sound at a higher frequency (or pitch) , similarly when the source of the sound is moving away from you or else if you are moving away from the source of the sound , you will hear the sound at a lower frequency ( or pitch). This is called Doppler effect.
Let us say the source of the sound is at a standstill and the sound emitted by the source is moving at a speed v , with the fequency of the sound being f_s and the wavelength of the sound being λ_s. You are moving towards the source at a speed v_a. The original frequency of the sound that you hear when you are not moving is f_s. Read more...➔

Cosmic microwave background radiation (CMBR) is a revealing informative residual from the very earliest time of the universe , almost from the birthing time of the universe. It was first detected in 1964. Its timeless existence is an irrefutable evidence that the universe originated from an extremely hot compressed point-sized origin or beginning. At the outset, Universe must have consisted of extremely hot radiation of plasma, elementary particles, photons (electromagnetic radiation) so as to have produced the CMBR which has almost uniform temperature and wavelengths in the radio region of the electromagnetic spectrum. So There were no galaxies, stars, planets or any other objects in the Universe in the beginning. Read more...➔

The spreading of galaxies is much more irregular and rugged per volume of space , fluctuations almost reaching more than 100% even though the cosmic microwave background radiation (CMBR) shows a uniform distribution of the radiation in the early Universe. The sky survey maps of the galaxies show narrow long thin spatial filaments connecting the collections of small and large galaxies and separated by voids that span billions of kilometers in the almost spherical form. The further most spread of galaxies in the Universe , the entangled connections between clusters , filaments and voids is what is the larg-scale structure of the Universe Read more...➔

Physicists came to know that they can not do the authentic tracing of the true density of matter in the Universe using only the galaxy maps generated by redshift surveys because matter did not accumulate enough at all regions to form galaxies to glow and be visible. So matter that has not aggregated enough to form galaxies may still exist today but is not visible. That is the reason why redshift maps are not sufficient to trace out the density of matter in the Universe.
Any matter whether that is visible or not exerts gravity on the surroundings. So this gravity of all matter creates cosmic flows which allows for estimating the density of matter in the Universe. As galaxies blow off out of the cosmic voids and gatther together into clusters and superclusters, they create irregularties in the expansion of the Universe and these irregularities are nothing but the "cosmic flows" Read more...➔

Galaxies gather together into large groups called "Galaxy Clusters" which are clumped together by their own gravity and are the largest objects in the universe . They comprise a lot of things and objects in huge numbers ⇒ hundreds or thousands of galaxies, lots of hot plasma, and a large amount of invisible dark matter. For example, the Perseus Cluster holds together more than a thousand galaxies and is one of the most shining sources of X-rays in the sky. Galaxy clusters are a domicile to the largest galaxies in the surveyed universe, and furnish the physicists with the information about the structure of the universe on the largest expanses.
The gas-dust and the stars in the galaxies clusters contributes only about 5% of its total mass. And besides, almost 80% of the mass of a galaxies cluster exists as the dark matter, and this dark matter is what clumps everything else together. The enormous amount of dark matter which the galaxy cluster is composed of generates enough gravity to curve the path of light traveling by. Read more...➔

Most of the evidence for dark matter is gravitational , which means it can be detected only through its gravitation effects on the surrounding objects. The divergence between the lucent matter and the matter exerting the gravitational force gives an evidence for the existence of a enormous undetected (invisible) mass in the Universe.
When physicists need to estimate the gravitational mass of a galaxy or of a galaxy cluster , they analyze the movement (motion) of a galaxy and apply the gravitational computations to approximate the gravitational mass needed to keep the system confined, similarly to the way that the gravitational attraction between the Sun and the Earth equilibrates the movement (motion) of the Sun and the Earth about the Sun. Read more...➔

The crooking of light when it is traveling through in the region of a gravitational mass brings about the lensing effect. This happening is called "gavitational lensing". It is a straightforward and candid effect of general theory of relativity put forth by Einstein which says that gravity governs the geometry of the space. Light travels in such a crooked path in the region of a gravitating body, resulting in the lensing effect. The beholder in the prospect of such a lensing mass may get to find or see twisted or quite a many images of an object that may exist in behind of the gravitating mass at a reasonable distance. The gravitating mass functions as a lens in this manner to the light being emitted from a background object.
Astronomers observed such lensing effects without any evident indication of lambent mass that can produce such lensing of the background objects. So it points at the existence of an enormous undetectable matter or dark matter. Read more...➔

Though we dont know the precise nature of the dark matter, according to its various gravitational evidence and our cosmological knowledge, we can assort it based on its possible formation (thermal or non-thermal) or based on their constituents' particles nature or according to the mass of the particles constituting the dark matter .
We can categorize the dark matter depending on the way it got created (either thermally or non-thermally) in the early Universe. When it comes to the thermal production, cosmic plasma generates dark matter via colliding during a epoch that is dominated by radiation. When it comes to the non-thermal production, other processes like the decaying of some heavy particles or specific symmetry contexts that exclude thermal formation may produce the non-thermal dark matter particles. Read more...➔

Planets and stars differ in a few ways

• Planets are cold and lustreless
• Planets are not as enormous as stars in size
• Stars are hot and emit visible radiation.

Apart from the above differences, there is one more that is extremely more significant. Planets are observed to have a fundamentally static stability. There is an everlasting balance between the inward-pull created by gravity and the outward-push generated by the electromagnetic fields of compact atoms. To the point that we know, a planet can keep up itself on its own as long as there is no external intervention.

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Physicists have predicted the existence of black holes for more than 200 years. They were just a philosophical idea at first for many years, but now there is a strong evidence that most if not all, galaxies comprise black holes millions or billions times more massive than our Sun. It is impossible to view black holes themselves as not even light can escape from the clutches of gravity inside a black hole. But physicists can analyze the effects of black holes on their surroundings.

Anything that crosses the event horizon, which is the border between a black hole and the outside world, is stuck inside. Not even light can get away quickly enough. Because of the tremendous gravitational pull of the black hole, nothing can escape from it. When big stars reach the end of their lives, black holes can develop that are a few times as massive as the Sun . Nothing can stop a star from collapsing onto itself , when all of its nuclear fusion sources have been used up.

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