Universe: Stars And Galaxies
6th Edition
ISBN: 9781319115098
Author: Roger Freedman, Robert Geller, William J. Kaufmann
Publisher: W. H. Freeman
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Chapter 25, Problem 36Q
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The difference between the
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Chapter 25 Solutions
Universe: Stars And Galaxies
Ch. 25 - Prob. 1QCh. 25 - Prob. 2QCh. 25 - Prob. 3QCh. 25 - Prob. 4QCh. 25 - Prob. 5QCh. 25 - Prob. 6QCh. 25 - Prob. 7QCh. 25 - Prob. 8QCh. 25 - Prob. 9QCh. 25 - Prob. 10Q
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- Why can't we see past the cosmological horizon? The universe extends only to this horizon. Beyond the cosmological horizon, the Universe curves away from us, like the surface of the Earth. O We do not have telescopes big enough. Beyond the cosmological horizon, we are looking back to a time before the universe had formed.arrow_forward1. The current (critical) density of our universe is pe = 10-26kg/m³. Assume the universe is filled with cubes with equal size that each contain one person of m = 100kg. What would the length of the side of such a cube have to be in order to give the correct critical density? How many hydrogen atoms would you need in a box of 1 m³ to reach the critical density? The matter we know, which consists mostly of hydrogen, constitutes only 4.8% of the current critical energy density of our universe. So how many hydrogen atoms are actually in a box of 1 m3 in our universe? Deep space is very empty and a much better vacuum than we can obtain on earth in a laboratory.arrow_forwardAssume a flat FRW universe, dominated by radiation throughout its history. Calculate the physical, cosmological horizon size at t=t_0 The initial conditions for the scale factor are a=a_0 at t=t0(today). Which asnwer is correct; a)R_H=2a_0ct_0 b) R_H = 3a_0ct_0 c)R_H = ct_0 d) R_H=2ct_0arrow_forward
- How do we know the whole universe is expanding?arrow_forwardWhat is the difference between hot dark matter and cold dark matter? What difference does that make to cosmology?arrow_forwardIf the universe had a density equal to its estimated critical density of 9= 10-30 g/cm3, and if it were composed entirely of one-solar-mass stars (mass 2.0 x1030 kg) distributed uniformly across the universe, what would be the distance between stars? Compare your result with the density of stars in the neighborhood of the sun and comment on the result.arrow_forward
- The present number density of electrons in the Universe is the same as that of protons, about 0.2 m-3. Consider a time long before the formation of the microwave background, when the scale factor was x times smaller than its present value. What was the number density of electrons then? Value: x = 12×106arrow_forwardThe CMB contains roughly 400 million photons per m3. The energy of each photon depends on its wavelength. Calculate the typical wavelength of a CMB photon. Hint: The CMB is blackbody radiation at a temperature of 2.73 K. According to Wien’s law, the peak wave length in nanometers is given by max=3106T . Calculate the wavelength at which the CMB is a maximum and, to make the units consistent, convert this wavelength from nanometers to meters.arrow_forwardExplain what we mean when we call the universe homogeneous and isotropic. Would you say that the distribution of elephants on Earth is homogeneous and isotropic? Why?arrow_forward
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