The size of the universe we a part, is not scientifically known but still we all know about the COSMOS all around us and have explanations about its dimensions in the scriptures. The sheer scale of the cosmos is hard to imagine, and even harder to put an accurate figure on. But thanks to some ingenious physics we now have a good idea of just how big it is. It took centuries but we now know the size of the universe. When we leave the solar system, we find our star and its planets are just one small part of the Milky Way galaxy. The Milky Way is a huge city of stars, so big that even at the speed of light; it would take 100,000 years to travel across it. All the stars in the night sky, including our Sun, are just some of the residents of this galaxy, along with millions of other stars too faint to be seen. Beyond our own galaxy lies a vast expanse of galaxies. The deeper we see into space, the more galaxies we discover. There are billions of galaxies, the most distant of which are so far away that the light arriving from them on Earth today set out from the galaxies billions of years ago. So we see them not as they are today, but as they looked long before there was any life on Earth. No one knows if the universe is infinitely large, or even if ours is the only universe that exists. And other parts of the universe, very far away, might be quite different from the universe closer to home. Future missions will continue to search for clues to the ultimate size and scale of our cosmic home.
Towards the full exploration of the size of our universe, beautiful images and straight-forward methods and ideas take us from our solar system, into the realm of the stars, the galaxies and finally into the vast panorama of the observable universe. Finding the distance to these very distant galaxies is challenging, but astronomers can do so by watching for incredibly bright exploding stars called supernovae. Some types of exploding stars have a known brightness - wattage - so we can figure out how far they are by measuring how bright they appear to us, and therefore how far away it is to their home galaxy. The further away a star is, the fainter it looks. Astronomers use this as a clue to figure out the distance to stars that are very far away. But how do you know if the star really is far away, or just not very bright to begin with? This problem was solved in 1908 when Henrietta Leavitt discovered a way to tell the 'wattage' of certain stars that changed their pulse rate linked to their wattage. This allowed their distances to be measured all the way across the Milky Way.
Throughout history, humans have used a variety of techniques and methods to help them answer the questions “how far” and “How big” is our universe. Generations of explorers have looked deeper and deeper into the vast expanse of the universe. And the journey continues today, as new methods are used, and new discoveries are made. As technology has evolved, astronomers are able to look back in time to the moments just after the Big Bang. This might seem to imply that the entire universe lies within our view. But the size of the universe depends on a number of things, including its shape and expansion. Ever since Copernicus argued that the Earth was not the centre of the Solar System, it seems we have always found it difficult to rewrite our preconceptions of what the Universe is – and especially, how big it may be. Even today, as we will see, we are gathering new evidence to suggest the whole Universe may be much bigger than some have recently thought. Today we are fairly confident that the Milky Way is probably between 100,000 and 150,000 light years across. The observable Universe is, of course, much larger. According to current thinking it is about 93 billion light years in diameter.
Astronomers have developed an ingenious array of tools and measuring systems to calculate not just the distance from Earth to other bodies in our Solar System, but the spans between galaxies and the journey to the edge of the observable Universe itself. The steps to measuring all these things are known as the "cosmic distance ladder". The first rung of the ladder is easy enough for us to get onto and these days it relies on modern technology. We can just bounce radio waves off of neighboring planets in the Solar System, like Venus and Mars, and measure the time it takes for those waves to come back to Earth and that gives us a very precise measurement. Big radio telescopes like Arecibo in Puerto Rico can do this sort of work – but they can also do even more than that. Arecibo, for instance, can detect asteroids flying around the Solar System and even produce images of them based on how radio waves reflect off the asteroid's surface. But using radio waves to measure distances beyond our Solar System is not practical. The next rung on the cosmic distance ladder is something known as parallax measurement.
Scientists measure the size of the universe in a myriad of different ways. They can measure the waves from the early universe, known as baryonic acoustic oscillations that fill the cosmic microwave background. They can also use standard candles, such as type 1A supernovae, to measure distances. However, these different methods of measuring distances can provide answers. It was Edmund Halley, famous for predicting the return of the comet that bears his name, who three centuries ago found a way to measure the distance to the Sun and to the planet Venus. He knew that the planet Venus would very rarely, every 121 years, pass directly between the Earth and the Sun. The apparent position of the planet, relative to the disk of the Sun behind it, is shifted depending on where you are on Earth. And how different that shift is depends on the distance from both Venus and the Sun to the Earth. It was knowing this fundamental distance from the Earth to the Sun that helped us find the true scale of the entire Solar system for the first time.
In 2013, the European Space Agency's Planck space mission released the most accurate and detailed map ever map of the universe's oldest light. The map revealed that the universe is 13.8 billion years old. Planck calculated the age by studying the cosmic microwave background. The cosmic microwave background light is a traveler from far away and long ago. When it arrives, it tells us about the whole history of our universe. Centering a sphere on Earth's location in space might seem to put mankind in the center of the universe. Because of the connection between distance and the speed of light, this means scientists can look at a region of space that lies 13.8 billion light-years away. Scientists know that the universe is expanding. Thus, while scientists might see a spot that lay 13.8 billion light-years from Earth at the time of the Big Bang, the universe has continued to expand over its lifetime. If inflation occurred at a constant rate through the life of the universe, that same spot is 46 billion light-years away today, making the diameter of the observable universe a sphere around 92 billion light-years.