Expanding of Universe And Discovery of Dark Energy

DISCOVERY OF DARK ENERGY

our universe is actually expanding!

Before starting this topic ,I would like to introduce you to some basic terms that we would use in this topic .

1. Redshifts- The displacement of spectral lines towards longer wavelength in radiation from distant galaxies and celestial objects. This is interpreted as a Doppler shift which is proportional to the velocity of recession and thus to distance. 

2.  Stellar object-Young stellar objects denotes a star in its early stage of evolution. This class consist of two groups of objects: protostars and pre-main-sequence star. 

3.  Supernova- A star that suddenly increase greatly in brightness because of a catastrophic explosion that ejects most of its mass.

4.  White dwarf-A star that has finished his life cycle and where nuclear fusion has stopped.

Einstein’s field equations, born out of his theory of general relativity gave scientists a tool to study the universe on the whole and make predictions. The equations had multiple solutions, some of which also corresponded to an expanding universe. The expanding universe propositions were simply looked as a theoretical exercise and not given much attention. That is until Edwin Hubble came along and provided evidence that suggested that the universe was not static after all. It was expanding !

Hubble measured the redshifts of some distant galaxies and also calculated their distances from the earth by measuring the apparent brightness of some stars in the galaxies. When he plotted  these obtained  redshifts as a function of distance from the earth he obtained a linear fit. 

This suggested that the redshift and the relative speed of a galaxy increased linearly as did its distance from the earth. It was a clear indicator and the first hard evidence that supported an expanding universe. Galaxies are moving away from each other and those that are further apart are moving away faster. It was a seminal discovery, one that changed the face of cosmology forever. Imagining the expansion of universe is no mean task. A raisin bread analogy is often used to help visualize the expansion of the universe in three dimensions. Think of initially, the raisins to be galaxies. Once the bread begins to bake and expand , the raisins move away from each other. But they move away from each other because the bread between them expands and not because of their own movements. Basically the galaxies are not speeding away from each other, the space between them is. Now the universe is made up of dense matter that exerts gravitational pull on other matter. This pull acts as an opposing force to the expansion ‘force’.

Think of a ball thrown in air. The initial momentum imparted can be imagined as a source of expansion. The ball will lose velocity on its way up because it has mass and hence suffers gravitational deceleration. Similarly, gravity was believed to be slowing down the universe’s expansion. To study this slowing expansion scientists needed to capture light from a very distant stellar objects since the light coming from these objects would essentially be from the past. The redshift of such an object would be a marker of the recession speed of the object in the past, when the universe was thought to be expanding at a much faster rate than what it is today.

For measuring distances of very distant galaxies astronomers needed a stellar body that is bright and has a standard luminosity. Such objects are called standard candles. The flux of light decreases according to Newton’s inverse square law. By making some required corrections and measuring the apparent brightness using a telescope, we can use the formula provided by the inverse square law and calculate the distance to the object.  Imagine a bulb kept at x meters distance from you. If you know the wattage of the bulb and are able to find the brightness perceived by you using some instrument. You can calculate the value of x.

It was found that a class of object known a type 1a supernovas could serve as good standard candles. Type 1a supernova occur in binary star systems when a smaller white dwarf attracts mass from the large star in the binary system, becomes unstable and explodes. The process of formation of a supernova explosion is well understood and their variations in luminosity occurs on human time scale in a predictable fashion. 

These supernova can occur in very distant galaxies and are bright enough to be detected by earth bound telescopes. This offers scientists an opportunity to probe events that happened early in the history of the universe. Two rival groups began to observe these supernova explosions and make precise measurements. The aim was to catalog as many explosions and hence plot as many points on the humble diagram as they could, preferably for the galaxies far away. One group was made up of physicists led by Saul Perlmutter  of the lawrence berkeley national laboratory and the other was a large collaboration of physicist and astronomers led by Brian Schmidt. The teams were called the SUPERNOVA COSMOLOGY PROJECT AND THE HIGH X SUPERNOVA SEARCH respectively. Both teams searched for very distant type 1a supernova and measured their apparent brightness and hence their distance. Using time on a hubble space telescope a spectral analysis of supernova could be done and the corresponding redshifts and hence the velocity could be measured. The distance and velocity  obtained would correspond to a point on the hubble space diagram. The hypothesis was that these points would lie below the straight line at 45 degree on the hubble diagram. But what both teams find was quite opposite. The points was above the straight line! it meant that back in the past the recession speeds of galactic objects was actually lesser than what current expansion rate suggested. The universe expansion had increased over time! The universe far from static and quite opposite to the decelerating model suggested, was actually accelerating!

It was an amazing and quite unexpected discovery. The discovery while dismissing many notions of our understanding of the universe suggested the presence of a ‘Dark’ mysterious force that counteracted the effects of gravity causing the expansion rate of the universe to increase. The energy that this force lead to was dubbed as ‘Dark energy’ .       

If you was unable to understand the article then read it one more time because after reading it first time I was also confused. But my confusion was cleared when I read it twice. So, just go for it .

Stay connected for some more unbelievable but true facts and theories of this unimaginable UNIVERSE .