Assignment 03 - Science PDF

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ACTIVITY 01 – SCI 1o Franz Kylle H. Pocson 201810968 BSA III SEC 41. State Hubble’s Law in your own words, paying attention to the definition of the parameters (i., consider relative to what locations is the distance measured).It is the observation in physical cosmology that galaxies are moving away...

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ACTIVITY 01.1 – SCI 1o Franz Kylle H. Pocson

201810968

BSA III

SEC 4

1. State Hubble’s Law in your own words, paying attention to the definition of the parameters (i.e., consider relative to what locations is the distance measured). It is the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance or the distance to a given galaxy from the observer is proportional to the recessional velocity. This means that from our perspective, everything is moving away from us in speed at which they are moving is different; closer things are moving away from us slowly while the farther things are moving away from us faster. This eventually led to the conclusion that universe is expanding. 2. Describe how Hubble measured distance and velocity. Hubble measured the distance from the apparent brightness of their stars, and from their galactic brightness — also known as the standard candle method, with the dimmer galaxies being farther. Hubble was fortunate to use the most powerful telescope in the world at that time, the 100-in. Hooker telescope at Mount Wilson, which enabled him to identify individual stars in galaxies and thus reveal their distances. How can light from the night sky tell us that the universe is growing in size? The main clue comes from something called redshift. Velocity can be measured by the redshift due to the Doppler effect. 3. Sketch a caricature or diagram of the situation or phenomenon showing that Hubble’s Law strongly implies an expanding universe.

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The sound of a passing car or in this case, an ambulance is a good example to explain the Expansion Phenomenon through the Doppler Effect. We know that when a vehicle is approaching you the pitch is high, and it gets lower as it passes you and recedes into the distance. The explanation is that the sound waves have a fixed wavelength (distance between two crests or two troughs) only if you're not moving relative to the source of the sound. If you are moving away from the source (or

equivalently it is receding from you) then each crest will take a little longer to reach you, and so you'll perceive a longer wavelength. Similarly if you're approaching the source, then you'll be meeting each crest a little earlier, and so you'll perceive a shorter wavelength. The same principle applies for light as well as for sound, distant galaxies are moving away from us extremely fast. So fast in fact that it is relatively easy to measure the shift in their spectral lines. This is evidence that the Universe is expanding, which is one of the most important pieces of evidence in support of the Big Bang picture. 4. Describe how Hubble determined Hubble’s Constant. What interesting specific information can be derived from it? The Hubble Constant is the unit of measurement used to describe the expansion of the universe. Hubble’s constant is also the slope of the line produced from the Hubble Diagram (graph of velocity vs distance) and this can be used to derive the age of the universe which is the inverse of the Hubble’s Constant, the Hubble Time. 5. How does Hubble’s Constant depend on time? How about distance? (In what sense is it constant?) Hubble’s constant decreases with time as the expansion rate of the universe decreases with time. As the distances between objects in the universe increase, expansion rate decreases thus Hubble’s constant also decreases. Hubble’s constant is only an average for this time. The reason we call it the Hubble constant is because the Universe expands at the same rate at every location in the Universe: the Hubble constant is constant only throughout space. 6. Does Hubble’s Law have anything to say about the motion of stars relative to other stars in the same galaxy? If yes, what is it? Yes. There is a caveat that Hubble's Law only works for distant galaxies. For nearby galaxies (in the Local Group), stars inside the Milky Way, and for objects in our Solar System, the relationship between distance and velocity does not hold. The reason for the discrepancy for nearby galaxies is the "peculiar velocity" of the galaxy, that is, its real velocity through space that is unrelated to the expansion. For distant galaxies, their peculiar velocities are small enough that they still lie on or near the line for Hubble's Law. For nearby galaxies, though, their peculiar velocity is larger than their velocity from the expansion, so their peculiar velocity dominates their total velocity, causing them to lie far from the line relating velocity to distance. 7. What is the error in Hubble’s observation? Using the distinction between accuracy and precision to support your argument, why is the error insignificant? Hubble’s values for his distances in 1929 were wrong, by a large factor of 7. This was mainly due to a wrong zero-point calibration of the standard candles used at the time. All distances were too small by a factor of 7, and the expansion rate Ho too large by the same factor. However, despite this large difference and its major implications for the expansion rate and age of the universe, Hubble’s fundamental discovery of the expanding universe is not affected as far as accuracy and precision is concerned; the underlying linear v ∼ d relation remains unchanged. Thus said, errors can still lead to

progress. The errors made on the way to the discovery of the expanding universe by the pioneers of the modern cosmology did not prevent the cosmological community from “getting it right” eventually. In fact, they were crucial steps in our progress toward understanding the cosmic expansion. Science progresses by correcting errors, not by avoiding them altogether. 8. If Hubble were in a different planet or galaxy, will he make the same observation described in Hubble’s Law? Yes he would, because the universe is expanding on all sides. The Earth and the Milky Way are not special in seeing that all galaxies appear to be moving away from us. If we were on a different galaxy, we would also see all the other galaxies appear to be moving away from us because of this expansion. 9. Enumerate a few other discoveries that are consistent with Hubble’s discovery and which support the occurrence of the Big Bang. Over the decades since Hubble’s discovery, numerous observations of the Hubble Law have been carried out to much greater distances and with much higher precision using a variety of modern standard candles, including Supernovae type Ia (SNIa) and a greatly improved stellar/Cepheid distance indicator to the Virgo cluster carried out with the Hubble Space Telescope, aptly named in honor of Hubble. But major discoveries include the Hubble Law, the Hubble Constant, the Hubble Time, and the more recent Hubble Space Telescope as tributes to this scientific breakthrough. 10. What are the particular “twist and turns” in the story that make Hubble’s discovery a dramatic story? Dramatic maybe is not the appropriate term to use, it is a actually commendable feat of overcoming a scientific adversity about the lack of evidential instruments in proving a theory. Thus Hubble’s discovery portrays such surprising scientific tale: from the large systematic error by a factor of seven in the measurement of distance, his velocities came mostly from those measured by Slipher, he used a small sample of merely 24 nearby galaxies, and his interpretation of the results in terms of the de Sitter kinematic model was wrong; yet, his main result of the velocity vs. distance relation changed the course of science by revealing the expanding universe....