About 15 billion years ago marked the origin of the universe, which started with a cosmic bang, termed the Big Bang. It’s the idea that tells the universe began just as a single point then expanded & stretched as large as it’s right now.
The bang not only created the majority of the elements in the universe but also created the physical laws that govern our ever-expanding cosmos.
The universe is expanding continuously, and scientists have gradually proved this scientific theory using systemic observations and scientific experiments.
Renowned astronomer Edwin Hubble was the first one to affirm the fact that the universe has been expanding, which he termed the Hubble law. He stated that galaxies are moving away from us with velocities that are proportional to their distances. In other words, since the Big Bang, the universe has been expanding, and the space between all the galaxies has been increasing. He concluded this by iterating over the work by Vesto Slipher, who earlier studied the measurements of radial velocities for galaxies. Slipher stated that cosmic bodies emit light that can be split into its component colors ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet) on a visible spectrum, the range of wavelengths (the distance between two successive crests or troughs of a wave) of electromagnetic radiation that our eyes are sensitive to. He noticed and that if the light source is moving away, the lines are shifted into the red part of the spectrum. This phenomenon occurs because the light from the faraway galaxies is stretched to longer wavelengths, and red color has longer wavelengths, which means they are redshifted, thus impart red color. This observation was the one that was important to understand the expansion of the universe and to tell the fact that the farther a galaxy is, the faster it is receding from Earth and the redder more blurred it would appear. However, Slipher never put the whole puzzle together, and this is where Hubble came into the picture, who properly theorized and proved the expansion of the universe using the same observations.
The universe has been expanding since its start and all the intergalactic material is moving apart from each other. However, there are galaxies that appear blueshifted i.e. if the light source is moving towards us the wavelength shortens, and blue color has a shorter wavelength, which means they are blueshifted, thus impart blue color. For instance, galaxies in the Local Group, a group of more than 20 galaxies around the Milky Way (including itself), appear to have blueshifted from on earth. Some galaxies are coming towards us, as there is gravitational pull exerted by the galaxies on one another. Gravity, which is the force of attraction that exists between any two masses, works on all scales. For instance, the Milky Way and the blueshifted galaxy are exerting a pull on one another, and this pull exerted by both the galaxies on each other overcomes the expansion rate, which is now considered about 69.8 km/sec/Mpc. The local gravitational attraction between galaxies can overcome the general expansion of the Universe, which is why some galaxies show blueshift rather than redshift. For instance, Andromeda (one of the galaxies in our Local Group) is approaching the earth at a high velocity, it will have a slight blueshift that is greater than that of the universe expansion rate. Gravity causes smaller galaxies to move toward larger ones, which in turn decreases the distance between galaxies, and that also makes the wavelength shorter and their color blue.
Hubble measured redshifts of a number of distant galaxies by using observing the “Standard Candles”, objects located within the universe for which their intrinsic luminosity, brightness, is known. He again iterated over the relationship, found by Henrietta Leavitt around 1911, between Period and Luminosity.
Leavitt observed & recorded the change in luminosity over time of cepheid stars, which was later more deeply looked at and publicized by Hubble. He also looked at the cepheid variables and calculated the distance of those and galaxies far away from them using the method of parallax, a change in position of an object when viewed from a different position. He observed the specific distant stars and calculated their distances when earth orbited around the sun. When he figured out the distances he plotted it against the redshift and found out a linear relationship between the distance and the redshift of distant galaxies, which tells that the universe is expanding. This also proves the fact that the universe must have been much smaller in the past, perhaps a single point, which is the idea of the Big Bang theory.
The expansion of the universe has been proven correct by Edwin Hubble, but he didn’t know how fast the universe has been expanding. He believed that the expansion rate of the universe, was fixed, meaning galaxies twice as far away from us should be moving about twice as fast away from us. There is a recessional velocity of a particular cosmic object, which tells at what rate that object is receding from the observer because of the expansion of the universe. Hubble found out that the recession velocities (v) of distant galaxies are proportional to their distances, and formulated Hubble’s Law equation :
However, until recently with the deployment of the Hubble Space Telescope (HST), we did find the rate at which the universe has been expanding, which is termed as the Hubble constant, which was previously considered about 70 km/sec/Mpc. HST, along with observing the cepheid variable, observed supernovas to calculate the farther distances, which could measure distances of about a thousand million light-years away, as well as redshift technique which could measure distances of about a billion light-years.
Through HST, astronomers learned that not only is the universe expanding, but it’s also accelerating, and they have been able to help calculate the age of the universe. Astronomers have assembled an image of pictures taken by HST called the Hubble Deep Field image (cover image), which comprises galaxies that perhaps may have emerged when the universe was young, which was around 14 billion years ago. Astronomers use light to study objects that are far away, as light enables us to look back in time. The most distant galaxies are over 13 billion light-years away, and we see them when they were pretty young, as the light takes 13 billion years to come to us from that galaxy. It would take another 13 billion years to see what that galaxy looks like right now.
Another evidence that has been supporting the expansion of universe theory has been the cosmic microwave background (CMB), which tells us about the age and composition of the universe through microwaves. It also imparts the knowledge of radiations, the heat, leftover from the Big Bang. It’s the evidence that shows that when the universe was born and since then it has undergone rapid inflation and expansion. Moreover, recently European Space Agency’s Planck used the CMB to calculate a more accurate value for the Hubble constant, which is 69.8 km/sec/Mpc.
Even though our universe has been expanding, it is the gravity that has held them together, the gravitational force has been shaping the large-scale structures in the universe like galaxies and galaxies clusters.
The expansion pushes away the galaxies from one another, but if the mass is enough of these galaxies is high enough, then gravity could halt the expansion in that particular region. This is also the reason why planets are orbiting around the Sun without drifting apart from it. We are also part of the galaxy cluster, called Local Group, where galaxies are attracting one another because of their masses. We shouldn’t be worried about the merging of the galaxies as this is not going to happen for another four billion years. Unfortunately, there will not be any life on Earth because the Sun would be nearing the end of its life, and have grown in size, and Earth would have got too hot for any life to survive, the oceans would have evaporated, and so on. Also, the space between stars is huge in the galaxies, so stars from the other galaxy wouldn’t be in danger of colliding with one another, the galaxies will merge rather than crash with one another.
Time and again, through constant observations and scientific process, we have verified the scientific theory that the universe has been expanding. Galaxy cluster density has shown variations, which are observed by the Sloan Digital Sky Survey that tells the evolution and expansion of the universe. Over the course of time, with the help of systemic calculations, experiments, and advanced technologies, astronomers and scientists have been able to rectify the fact that the universe has been expanding and this expansion has been accelerating.
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