![]() The expansion history of the Universe (figure 2), and thereby to place constraints on the properties of dark energy. The Hubble observations relied on a particular type of supernova explosion (known as Type Ia supernovae) to trace Since the discovery, observations of the cosmic microwave background radiation have shown that dark energy makes up more than 70% of the total energy density of the Universe. Of the dark energy that powers the acceleration is arguably the biggest puzzle that physics is facing today. This came as a shock, because the prevailing assumption was that cosmic expansion should be slowing down due to the mutual gravitational attraction of all matter within the Universe. In 1998, two teams of astronomers working independently discovered that the expansion of our Universe is in fact speeding up, propelled by the repulsive force of a mysterious dark energy. Most recently,ĭetailed HST observations of Cepheids in the galaxy NGC 4258 - whose distance is very accurately known through radio observations - coupled with observations of well-calibrated supernova explosions in more distant galaxies, have reduced the error in the value of H 0 to less than 5%. The superb resolution of HST has allowed astronomers to isolate the light from numerous Cepheids in dozens of galaxies (figure 1), and the uncertainty in the value of the Hubble constant has initially been reduced to about 10%. By measuring the period of the variation, astronomers deduce the intrinsic brightness, and byĬomparing that to the apparent brightness, they can determine the distance (the apparent brightness decreases like the inverse square of the distance). These are luminous stars, about a thousand times more luminous than the Sun, whose intrinsic brightness is tightly correlated to their pulsation period. Key standard candles are certain pulsating stars - stars that show periodic variations in their brightness - known as Cepheid variables. Relatively well known - to infer distances to objects that are increasingly farther away.įigure 1: Cepheid variable star in galaxy M100. To overcome this obstacle, astronomers have constructed a distance ladder in which they use a series of standard candles - objects whose brightnesses are The main reason for this large uncertainty was the fact that the determination of distances in astronomy is notoriously difficult. But even in the presence of deceleration and acceleration H 0 is the dominant factor in determining the age of the Universe.īefore HST, the value of H 0 was known only to within a factor of two. For a constant rate of expansion, the inverse of H 0 gives the time at which the expansion started. The rate at which galaxies are currently moving apart from one another is The fabric of space between any two distant galaxies is stretching, just like the rubber of an inflating balloon. The astronomer Edwin Hubble, after whom the Hubble Space Telescope is named, determined in the 1920s that our Universe is expanding. Nevertheless, in the topics that I have selected, there is no doubt that HST played a crucial role. ![]() Usually it is many observatories, from the ground and from space, working in concert to produce a complete and multihued view of phenomena. ![]() I should also note that rarely do astronomical discoveries belong to just one telescope. Order) what I regard as the top five scientific discoveries. In what follows, I have attempted to briefly describe (in no particular With such a record, it is clearly impossible to even just list the HST's scientific accomplishments in a short article. At the time of this writing, 7,917 refereed scientific articles have been written based on HST data. During its 19 years of operation, HST has produced 39 terabytes of data in the HST archive. This marked the beginning of the next phase in the life of this incredible On May 19, 2009, the Space Shuttle Atlantis released the Hubble Space Telescope (HST) back into orbit after a hugely successful servicing mission, during which two new instruments have been installed on board the telescope, and two existing ones have been repaired. ![]() Gas pillars in the Eagle Nebula: One of Hubble's most famous images.
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