THE EVER EXPANDING UNIVERSE – EXPLANATION OF THEORIES AND ITS SIGNIFICANCE

Whether the universe is expanding or is static was not known before the twentieth century. The belief in a static universe was strong till that time.

According to the most widely accepted theory for the origin of our universe, the Big Bang, it is believed that about 13.8 billion years ago, all the energy and matter was condensed into an infinitesimally small point probably smaller than an atom. The singular point then started expanding and created the universe we all are familiar with.

PAST CONCEPTS

The question whether the universe is still expanding or not remained mostly unanswered till the twentieth century. Till then it had been believed that the universe had remained the way it is since the time of creation.

Physicists adhered to the idea that the universe was static. They denied the possibility of expansion even though there had been contradictions to the theory of a static universe.

THEORIES RELATED TO EXPANDING UNIVERSE

In 1687, Sir Isaac Newton published his book Principia Mathematica. In this book he explained the laws of motion and the universal law of gravitation. His theories revolutionized the prevailing concepts related to the cosmos. Gravity, as Newton described is a force by which all particles in the universe attract each other. More massive bodies exert a greater gravitational force. Closer the object, greater the gravitational pull and vice versa.

But if gravity attracts bodies, then the celestial objects must not remain motionless by virtue of gravity. This implies that they should fall over each other under the influence of gravity. This raised an objection to the principle of static universe.

Another conflict with the static model of universe was put forward by German philosopher Heinrich Olbers. He stated that in a static universe, all the boundaries would merge with the surface of a star. The stars would therefore illuminate the sky brightly even at night and this would occur despite light’s absorption by intervening matter, as the matter would also heat up after absorbing light and glow bright.

The fact that sky is not bright at night can be explained by considering that stars have not been shining since the creation of universe but started emitting light afterwards. In that case, light from distant stars has not been able to reach us yet nor been able to heat up the absorbing matter. This may also be attributed to the concept of an expanding universe.

An expanding universe was found reasonable explanation to this paradox. The diminishing light intensity from faraway stars due to expansion of universe was a probable explanation to above paradox.

EVIDENCE FOR EXPANDING UNIVERSE

For a long time it was assumed that the universe consisted of just the Milky Way galaxy which is where our solar system is located. Only in 1924, it became understood that there are numerous galaxies like ours in the universe.

Edwin Hubble calculated the distance to nine galaxies based on their luminosity and brightness.

When light from a star passes through a prism, it splits into its spectrum. We can measure star’s temperature from its spectrum. In 1920s, astronomers studied the spectra of various stars in our and neighboring galaxies. They were surprised to find that the spectra formed by light emitted from other galaxies were all shifted towards the red side of the electromagnetic spectrum.

Let us take a break to try and understand a few concepts. The visible light of the electromagnetic spectrum has colors of different wavelengths between 400 – 700 nm. Violet has highest frequency and shortest wavelength, while red has lowest frequency and longest wavelength. ( wavelength is inversely proportional to frequency). Red shifting of light therefore means that the frequency of light is decreasing.

Scientists noted that this bears a similarity to Doppler Effect. Doppler effect is the phenomenon by which there is change in the frequency of sound perceived by an observer when there is relative motion between the source of sound and the observer. Sound waves emitted by siren of a moving ambulance seem to be of higher frequency or pitch when moving towards an observer and of lower frequency or pitch when moving away from observer.

Light spectrum of stars of other galaxies were shifted towards red end of spectrum which means that their frequency was decreasing ( that is wavelength increasing) due to doppler effect. This suggested that the galaxies were moving away from us.

In 1929, Edwin Hubble proved that larger the distance of the galaxy, the faster was its movement. The red shift was greater (in fact directly proportional) to the distance of galaxy from us. This simply means that galaxies that are farther from us are moving away from us at a faster rate.

This led to the discovery that the universe is not static but is expanding. Expansion of the universe was a remarkable scientific discovery of the twentieth century.

SIGNIFICANCE OF EXPANDING UNIVERSE

So what important things can we expect based on universe’s expansion. If universe has been expanding all the time does this not mean that all objects were closer in the past and even more closer before.

This led to the establishment that there must have been a time when all the matter was exactly at the same point. The singular point must have been of infinite density and then expanded to form the universe.

HOW LONG WILL UNIVERSE EXPAND

We now know that our universe is expanding by 5 to 10 % per thousand million years. But how long will it continue to do so. Considering the current rate of expansion, if density of universe exceeds a critical value, then gravity will become strong enough to stop the expansion and eventually lead to collapse. On the other hand, if density is less than the critical value, then universe will continue to expand.

The current density of the entire universe including density of dark matter makes up only one – tenth of the value needed to stop the expansion.

The available data, therefore, indicates that the universe will keep expanding.