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The universe is expanding because of the ''Big Bang''
The concept of an expanding universe simply means that the universe as we know it is getting bigger. "In 1929, Edwin Hubble showed that distant galaxies were, indeed, receding from the earth, and the further away they were,the faster they were moving. That discovery changed cosmology." Hubble, working at the Carnegie Observatories in Pasadena, California, measured the redshifts, (a redshift is the measurement of the light coming from distant objects), of a number of distant galaxies. He also measured their relative distances by measuring the apparent brightness of a class of variable stars called Cepheids in each galaxy. When he plotted redshift against relative distance, he found that the redshift of distant galaxies increased as a linear function of their distance. The only explanation for this observation is that the universe was expanding. Once scientists understood that the universe was expanding, they immediately realized that it would have been smaller in the past. At some point in the past, the entire universe would have been a single point. This point, later called the big bang, was the beginning of the universe as we understand it today.
An expanding universe implies some physical characteristics:
1. The bulk of atoms are made up of electrons moving about protons and neutrons ("Matter")
2. Gravity is attractive (Matter clumps together)
3. Time moves only forward
4. Red Shift is apparent when measuring large distances
5. Entropy (disorder) increases
6. Enthalpy (energy content) decreases
So in this universe, to use a classic example, in dropping a teacup gravity causes it to move towards Earth and smash to pieces (increase entropy) while time moves forward. In space, large clumps of matter (galaxies) are accelerting away from us (Red Shift) and energy content decreases, as the stars burn their nuclear fuel.
In contrast, a contracting universe is one where:
1. The bulk of atoms are made up of positrons moving about antiprotons and antineutrons ("AntiMatter")
2. Gravity is repulsive (Matter spreads out)
3. Time moves only backward
4. Blue Shift is apparent when measuring large distances
5. Entropy (disorder) decreases
6. Enthalpy (energy content) increases
In this universe, broken china shards on the floor reassemble into a teacup (decrease entropy) while moving away from Earth (Antigravity) while time moves backward. In space, Antimatter galaxies accelerate towards us (Blue Shift) dispersing into atoms, and energy content increases as uniform clouds of gas in this contracting universe are squeezed together.
"A Brief History of Time," Stephen Hawking, 1990
So, you want to know what is expanding universe?
There are two words -
expanding & universe.
universe means the cosmos where everything is there - the planets, the stars, the asteroids, the black hole etc.
And expanding means it is becoming big and big. Growing and growing. Spreading and spreading.
when you fill air in a baloon. thus it grow or expand.
it expands, so its the same thing to the universe/cosmos. Due to the big bang the cosmos is expanding. The big bang happened because the early universe was very very dense.
And now also the cosmos is expanding.
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