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If somehow we were able to get above the Milky Way it would look like an enormous spiral about 100,000 light years across. Let's try to understand red and blue shift. When an object is moving away from us the light from the object is known as red shift and when an object is moving towards us the light from the object is known as blue shift. This is because of the change of the wavelength. Astronomers use red shift and blue shift to deduce how far an object is away from earth. In physics a red shift is an increase in the wavelength and corresponding decrease in the frequency and photon energy of electromagnetic radiation such as visible light. The light emitted from distant galaxies appears red shifted in comparison with light emitted on the earth. In the light of Big Bang Theory about 13.8 billion years ago the whole universe was a very small extremely hot and dense region. From this tiny point the whole universe expanded outwards to what exists today. Astronomers have discovered that in journal the further away a galaxy is the more red shifted its lightest. This means that the further away the galaxies are the faster they are moving. This is similar to an explosion when the bits moving fastest travel furthest from the explosion. Redshift data provides evidence that the universe including space itself is still expanding. Microwave radiation of this specific frequency is observed at all point in space around us and it is known as cosmic microwave background radiation. This radiation was produced shortly after the universe was formed and that this radiation has been expanded into the microwave region of the electromagnetic spectrum as the universe expanded. As we already discussed that universe is expanding what does it mean? It means there should be a specific speed which a galaxy is moving away from the earth. It can be found from the change in wavelength of the galaxy's star light due to redshift. The distance of a far galaxy let's say D can be determined using the brightness of a supernova in that galaxy. In 1929 the astronomer Edwin Hubble showed that the universe was expanding. He did this by observing that the absorption line spectra produced from the light of distance galaxies was shifted towards the red one of the spectrum. This Doppler shift in the wavelength of light is evidence that distant galaxies are more moving away from the earth. Hubble also observed that light from more distant galaxies was shifted further towards the red end of this spectrum compared to the closer galaxies. From this observation he concluded that galaxies or a star which are further away from the earth are moving faster than galaxies which are closer. According to Hubble's law the recessional velocity V of a galaxy is proportional to its distance from earth. We can write down or express this in this kind of equation where H0 is equal to V divided by D. You can see on the screen this H0 is actually the Hubble constant. Don't worry the students in exam the value of H0 will be provided. The accepted value of the H0 is 2.2 time 10 rest to power minus 18 per second. Where you can see here this V is a recessional velocity of an object means the velocity of an object moving away from an observer and the unit is kilometer per second. This D is actually the distance between the object and the earth and the unit is kilometers. So if we summarize the Hubble constant so we can say that the H0 is the ratio of the speed at which the galaxy is moving away from the earth to the distance from the earth. We already discussed that this H0 is equal to V over D so we can also rearrange as 1 over H0 is equal to D over V. A key aspect of Hubble's law is that the furthest galaxies appear to move away the fastest. The gradient of the graph can be used to find the age of the universe. So when the distance equals zero this represents all the matter in the universe being at a single point. This is the singularity that occurred at the moment of the Big Bang. The units of gradients are per second the same as the unit of Hubble constant. By taking the reciprocal or 1 over H0 the units will become seconds. Therefore the reciprocal of the gradient represent time and gives the amount of time which the universe has been expanding for. Astronomers have used this formula to estimate the age of the universe which is about 13.7 billion years. So I think it's enough for today. So thank you very much for listening and watching. I hope you enjoyed the video. I hope this video is useful for you. So take care of yourself and don't hesitate to write an email if you have any question, query, suggestion and even classes are required. Take care of yourself. Meanwhile, see you in the next video. Goodbye.