expanding universe | will universe continue to expand? | how did we come to know universe is expanding? | Red shift | Doopler's effect

Earth is the center of the solar system. Everything revolves around the earth. Yeah! You read this right. This was the view of humans about the solar system before the 15th century. Since then our knowledge have been improved a lot. We now know that there are billions of stars in the milky way galaxy, there are millions of galaxies in the universe, there are crores of different universe in the space, and lots of other things. 

Almost a century ago, a discovery that shook the whole world and brought revolution in our understanding of 

the universe was that, universe is expanding.

The discovery was too revolutionary. Albert Einstein’s theory of relativity predicted that universe must be expanding, but the idea of static universe was so popular that he introduced an antigravity force to avoid the predictions. 

One big question which arises here is how do we know that universe is expanding? Humans have only been to the Moon and Our robotic missions are also limited within our solar system. Then How did 

scientists conclude that universe is expanding? There is no direct method to observe this phenomenon. Luckily, we have amazing group of scientists, they found some indirect methods to find this out. 

Newton found an amazing property of light. When light falls on the prism it gets split into its constituent colors. There is only one property which we need, to get much information about the planet, star and galaxy and that property is the apparent brightness of the light. Apparent brightness means the light we receive from the celestial bodies. It is called apparent because it is not the actual light that the body emits or in case of planets, reflects. We can tell: of what elements the star is made up of, how far is the galaxy from us and many more things by just knowing the apparent brightness. How?  Every element has its characteristic spectrum. So, when the light from the star falls on the prism, some part of the spectrum is missing (that part is absorbed by the element) which can be analyzed and the elements from which the star is made can be known. 

If we calculate the distance of various stars of a galaxy, we can tell that how far is that galaxy from us. This method was found by Edwin Hubble. Using this he found the distance of 9 different galaxies from us. He also concluded from the data that Universe is Expanding. How did he do That? Before answering this, we need to understand ….

Doppler's effect

You might have noticed one common thing, when a vehicle like car approaches to us, the sound of its horn heard, is of higher intensity than when it is going away from us. But when the same car is standing still, we notice no variation in the intensity of the sound of its horn. This is called  Doppler’s Effect. When the car approaches us, the frequency of the sound wave increases and opposite happens when it leaves us.

What’s the connection with expansion of universe?

The connection is that Doppler’s Effect is also observed for the light waves. When light wave is approaching us its frequency increases and hence its wavelength decreases. So, the light waves appear bluish to us. The opposite happens when light wave is going away from us and it 

appears reddish to us. This is what we call Red shift. Using this technique Edwin Hubble found out that most of the galaxies showed red shift. This means that almost all galaxies are moving away from us. He also noticed one shocking information that, the expansion increases with the distance of galaxy from us. It means that the farther the galaxy, more is the rate of expansion. This was a breaking news. This proves that universe is not only expanding but it is also accelerating (rate of expansion is increasing with time).

Why didn’t anyone before Hubble found this out?

After anything is discovered it becomes obvious. It is obvious fact that universe can’t be static. If universe were static, soon it would collapse due to the effect of gravitational force. But the idea of static universe was so misleading as well as popular that many scientists ignored this basic fact.

Will this expansion continue forever?

The answer to this question is not simple. There are 3 possible scenarios: -

1. Universe Expanding Slowly

If this is true then soon the gravitational attraction between the galaxies will slow down the expansion. The universe will begin to start contracting. 

2. Universe expanding rapidly

If this is true then the gravitational effect can never stop the expansion. The universe will continue expanding.

3.Universe is expanding just to avoid re-collapse

If this is true then the separation between galaxies must increase at a constant rate. Our observation say that this is not true. 

All these models were proposed by Friedmann. Now a question comes in our mind that Which one of these describes our universe? The expansion rate of our universe is increasing very slowly about 5% to 10% per billion year. There are 2 more scenarios: -

1. Density of universe is greater than the critical value (here, the deciding value which would decide the expansion or contraction rate) – if this is true then the gravity is weak and the expansion can’t stop.

2. Density is less that the critical value – if this is true then the gravity is strong and universe would re-collapse or contract. 

To find out the truth, we need to calculate the total mass of the universe. Scientists found this out and to our surprise the mass of all the matter present in our universe is too less than that required to stop the expansion. The mass is about 100 times less than the required mass. If we add all the dark matter than also the 

mass is about 90 times less than the required mass. So, from this scientist concluded that Universe will 

continue to expand. 

Friedmann published his theoretical results in 1922 and in 1929, Hubble proved it based on observation. 

Our knowledge about our universe is expanding day by day. More and more shocking truths are being uncovered. As far as our knowledge is concerned, we don’t even know about 5% of our universe. With the advancement in technology we have revealed many dark secrets about the universe and will soon find out some more secrets.

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