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Dec 6, 2023

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AST 309 Exam 2 NAME: EID: **The first 4 questions are 1 point each, questions 5–12 are 2 points each, and questions 13–14 are 5 points each.** 1. (1 point) Dark Energy is causing the universe to accelerate faster than expected. Before we discovered dark energy, we expected it to be decelerating, and there was even a cosmological parameter called the deceleration parameter. This parameter quickly disappeared when the measurement showed we are accelerating. What is the reason why all of the theorists believed the universe should be decelerating in the first place? GRAVITY 2. (1 point) Dark energy is causing spacetime to expand through your bodies. Why are we all not continually getting larger then? It doesn't make our bodies grow because electromagnetism and the nuclear forces that hold atoms and matter together are much stronger than the force of dark energy causing the universe to expand. 3. (1 point) Name the four components that we think are all we need to consider in order to explain how the universe is expanding. Radiation, matter, shape, dark energy 4. (1 point) The Andromdea galaxy is about 1 Mpc away from us. According to Hubble’s Law, what is the velocity at which space is expanding between us and Andromeda? Why is Andromeda coming towards us then? Hubble's Law describes the relationship between the distance of a galaxy and its velocity due to the expansion of the universe. The equation is Velocity = Hubble's Constant × Distance. Given that the Andromeda galaxy is about 1 Mpc away (1 Megaparsec = 3.09 × 10^19 km), and using an estimated Hubble's Constant of around 70 km/s/Mpc, the velocity at which space is expanding between us and Andromeda would be approximately 70 km/s Despite the expansion of the universe causing most galaxies to move away from each other, some, like Andromeda, are close enough that their gravitational interactions can overcome the expansion. Hence, Andromeda is coming towards us due to the gravitational pull between our galaxy (the Milky Way) and Andromeda. 5. (2 points) As discussed in class, provide a simple explanation for how our observable universe began.

6. (2 points) Design a model of gravity that explains both dark matter and dark energy. Make sure to state whether it would be more or less strong compared to Newton in various cases, and make sure to state the size scales on which these modifications would operate. 7. (2 points) Our current standard model of the universe is called ΛCDM. Describe each of the 4 symbols and where they come from observationally. 8. (2 points) What is beyond the observable universe? Put these in order from the most probable to the least probable: G. An infinite sea of spacetime continuum bubbling with quantum fluctuations R. A large shell of mass A. Nothing V. The edge of a black hole I. Prince T. Selena Y. King Gizzard and the Lizard Wizard 9. (2 points) If the universe increases in size by a factor of 2, what happens to the density of shape, density of matter, density of radiation, and density of dark energy? The Density of space would decrease. As the universe expands, the volume increases, leading to a lower density of space. The Density of matter would decrease. Since matter is distributed in the expanding space, when the universe increases in size, the density of matter decreases due to the same amount of matter being spread across a larger volume. Density of radiation would decrease. As the universe expands, the wavelengths of photons stretch out, resulting in a decrease in their density. Density of dark energy would remain constant. This enigmatic form of energy is thought to be an intrinsic property of space, meaning its density doesn't change as the universe expands.

10. (2 points) Explain in an expanding universe why the density of photons would decrease as the 4th power of the size. The density of photons decreasing as the 4th power of the size refers to the concept of radiation in an expanding universe. As the universe expands, the volume increases. The density of radiation, specifically photons, is inversely proportional to the volume. In an expanding universe, the volume increases as the cube of the scale factor, while the number of photons remains constant (since they are not created or destroyed). Therefore, the density of photons decreases as the fourth power of the size due to the increase in volume in the cubic relationship and the constancy of the number of photons. 11. (2 points) The cosmic microwave background is the residual heat from the Big Bang. If it was so big and hot, why are we not burned up today? The cosmic microwave background (CMB) has cooled significantly since the Big Bang, now existing at a chilly 2.7 Kelvin, making it detectable as microwave radiation rather than heat, and is spread thinly across the vast expanse of the universe, hence not posing a threat of burning or any significant thermal effect on our present-day existence. 12. (2 points) Sketch the power spectrum for how people are distributed on a school bus. Label the axis, and label what each peak corresponds to. 13. (5 points) Is the universe expanding faster or slower in the past? Assuming dark energy dominates, will the universe expand faster or slower into the future? Explain your reasoning in both cases. Draw a plot of the expansion rate versus time (label your axes), from the beginning of the universe to into the future. Make sure to specify where today is. 14. (5 points) Explain how Hubble used Cepheid stars to discover that the universe is expanding. Make sure to discuss the types of observations he used, and how each implies velocity and distance. Using Cepheid stars, Edwin Hubble revolutionized our understanding of the universe's expansion. By measuring the periods of Cepheids in Andromeda, he determined distances far beyond our galaxy's edge, indicating these objects were other galaxies, marking a pivotal shift in cosmology. Noticing these galaxies' radial velocities (how fast they moved away), he observed that nearly all were receding, leading to his groundbreaking 1929 discovery: the Hubble Law, where a galaxy's distance is directly linked to its velocity, revealing that the further a galaxy, the faster it recedes from us, demonstrating the universe's expansion. Hubble's original plot showcased this relationship, and the Hubble Space Telescope later refined Hubble's constant, vital for the velocity-distance equation, from an accuracy of 50% to around 5%, crucial in shaping our understanding of cosmic expansion dynamics.

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