The Hubble Space Telescope, arguably the greatest technological breakthrough of its time, now have over three decades of service. Throughout its prestigious existence, Hubble forged a distinguished path, deciphering ground-breaking discoveries and revolutionizing our understanding of the universe. Its pioneering discoveries not only contributed to the advancement of science but also fascinated a worldwide audience into astronomy, becoming an iconic symbol of long-lasting significance in the realms of human scientific explorations.

One of the greatest achievements of Hubble is to prove the expansion of the universe. Hubble observed a myriad of different galaxies and provided precious data for calculations. When the space between the galaxies expands, the wavelength of photons travelling through it is stretched and increased. This leads to a redshift of the electromagnetic spectrum. Redshift occurs when the whole electromagnetic spectrum is shifted slightly towards higher wavelengths side. This cosmological redshift is analysed and compared to find out the recession speeds of the individual galaxies. What we discovered was that all the galaxies are moving away from us at an accelerating rate, with an exception of a few neighbouring galaxies. Hubble’s law, named after Edwin Hubble, provided us with an equation for this expansion. Using data from the Hubble telescope, the equation was proved to be correct and a value for the Hubble’s constant was found.

The discovery of Dark energy was one of the achievements of Hubble. Before 1998, astronomers thought that the universe is decelerating because of gravitational pull. Data from Hubble revealed that the universe is expanding at an increasing rate. Since then, Hubble is studying dark energy by observing the speeds of different objects. Hubble was used to observe the distant supernovae. The supernovae are expected to dim over time but the rate of dimming was way more than it was expected. This led astronomers into believing that there must be a mysterious energy that is causing this acceleration. The origin of this energy is hence the name dark energy.

Hubble also helped us estimate the age of the universe. The value of Hubble’s constant is measured using the data from the Hubble Observatory. Hubble’s constant allows us to calculate the speed of recession of two distant objects. Mathematically, Hubble’s constant is the inverse of the time. This time is the time when both objects were at the same place. This could be interpreted as, the time when both objects were at the same place was that of the big bang. Now we just take the inverse of Hubble’s constant and get the time, in seconds, for the age of the universe. This time is about fourteen billion years. Although this method has some limitations, this is the best estimate of the age of the universe so far. Hubble was also used to study the atmosphere of exoplanets. Astronomers used a technique called the transit method to interpret the composition of their atmosphere. In this method, when an exoplanet is between the host star and the telescope, there is a slight dimming of light from the host star. This reduction in brightness could let us calculate the size of planets. If we study the light coming from the atmosphere of that planet, this provides us with an idea about the chemical composition of that atmosphere. The instruments onboard Hubble could identify molecules like water, methane etc. giving us a deep insight into similarities and differences with our solar system planets.

Gravitational lensing is a phenomenon where light bends around an object such that we can see an object that is behind the other object. Photons are massless so their path should be unaffected by gravity but the images from Hubble show they are indeed deflected by gravitational force. This is a direct proof of Einstein’s general theory of relativity. This proves that gravity bends the fabric of space-time so photons follow a curved path around the objects with enough gravitational force. One such example is the cosmic horseshoe. This is a distant galaxy, approximately 10 billion lightyears away, captured by Hubble. There’s another galaxy behind this one that could be seen as a distorted image, like a horseshoe, around the first one. Another example is Einstein’s cross where a quasar distorts the light from a galaxy behind it. There are a number of events where Hubble pushed its limits to capture extremely faint distant galaxies exhibiting gravitational lensing providing us with a deep insight into how the space-time curvature works.

There’s a supermassive black hole, Sagittarius A*, at the centre of our own Milky Way galaxy. Hubble’s observations revealed that this black hole is surrounded by a hot disk emitting radiations. This radiation was used to map the movement of the disk and the mass of the black hole was determined. The Event Horizon Telescope used the data from Hubble to calibrate the equipment to capture the image of a black hole. Project Event Horizon used images from Hubble and several ground-based telescopes to mimic a virtual earth-sized telescope. The result was a high-resolution image of a black hole in the galaxy M87 captured in 2019, the first-ever direct image of a black hole. Then in 2021, EHT captured another image of a black hole. This time the target was Sagittarius A*. This was the first clear image of Sagittarius A* making it one of the most emblematic Astro-images.

Hubble observed a huge number of distant and neighbouring galaxies allowing scientists to make a map of our cosmos unravelling the bigger structure of the universe. This allowed the scientist to study the distribution of galaxies to make conclusions about the future of the universe. Hubble revealed a diverse variety of galaxies billions of light years away unveiling valuable information about the formation of different galaxies. One of the finest images of extremely faint galaxies captured by Hubble was the ultra-deep field. Hubble’s ultra-deep field (HUDF) was an image of the sky that is around the size of a grain of sand held at arm’s length. Hubble observed this area of sky for about eleven days and the data was combined to produce a final image that is the deepest image of the universe ever taken. The image has around 10,000 different galaxies, a few of which are around 13 billion light years away. These galaxies formed 300 – 400 million years after the big bang. HUDF taught us that there were a greater number of galaxies in the early universe and the early galaxies were more active than modern galaxies. This spectacular masterpiece captured by Hubble is a great leap forward in our understanding of the universe. Hubble did not only capture farther galaxies but also closer galaxies in great detail. Hubble captured the most detailed image of the closest galaxy Andromeda. The image spans over billions of pixels and the size of this image is over four gigabytes. Hubble captured several other galaxies like pinwheel galaxy and sombrero galaxy in great detail revealing the structural details that helped us map our own galaxy.

Hubble played a vital role in the discovery of dark matter by observing the motion of galaxies. While observing clusters of galaxies, Hubble found out that the speeds of galaxies were more than expected hinting about an extra mass that needs to be there to account for higher speeds. Observing the rotational speeds of galaxies, it was revealed that the speed of rotation of a galaxy does not decrease with increasing the distance from the galactic nucleus (centre of the galaxy). This suggests the presence of undetected mass to satisfy our mathematical formulations. Gravitational lensing is another proof of the existence of dark matter provided by Hubble. The gravity of an object needs to be much more than the visible matter suggests for the gravitational lensing to be observed. All of the data from Hubble not only suggested the existence of dark matter but also mapped the distribution of dark matter throughout the universe.

The legacy of Hubble continues and it is not expected to retire any soon. Being in low earth orbit makes it easier for scientists to upgrade the onboard equipment and repair any defects through spacewalks. Hubble is strategically placed above the atmosphere so it has several advantages over ground-based telescopes. There will be no interference of air turbulence in the images, and shaking of the ground will not affect the results of Hubble allowing it to produce extraordinary results. The precise calibration of Hubble’s instruments is used as a reference for calibrating other telescopes. Even JWST is being calibrated using the data from Hubble. Hubble remains a great technological advancement, an indispensable tool for scientific advancement and a symbol of mankind’s endless space exploration!