*39 Will the universe continue to expand forever? To attack thisquestion, assume that the theory of dark energy is in error and thatthe recessional speed v of a galaxy a distance r from us is determinedonly by the gravitational interaction of the matter that lies insidea sphere of radius r centered on us. If the total mass inside thissphere is M, the escape speed v, from the sphere is v. = V2GMIT(Eq. 13-28). (a) Show that to prevent unlimited expansion, the aver-age density p inside the sphere must be at least equal toЗН87G(b) Evaluate this "critical density" numerically; express your an-swer in terms of hydrogen atoms per cubic meter. Measurementsof the actual density are difficult and are complicated by the pres-ence of dark matter.

Answered: Image /qna-images/answer/561aa1ff-4798-4adb-81e7-d8cbae4898aa.jpg

Answered: *39 Will the universe continue to…

Modern Physics

3rd Edition

ISBN:9781111794378

Author:Raymond A. Serway, Clement J. Moses, Curt A. Moyer

Publisher:Raymond A. Serway, Clement J. Moses, Curt A. Moyer

Chapter16: Cosmology

Section: Chapter Questions

Problem 11P

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(a) What Hubble constant corresponds to an approximate age of the universe of 1010 y? To get an approximate value, assume the expansion rate is constant and calculate the speed at which two galaxies must move apart to be separated by 1 Mly (present average galactic separation) in a time of 1010 y. (b) Similarly, what Hubble constant corresponds to a universe approximately 21010years old?
Explain why it only appears that we are at the center of expansion of the universe and why an observer in another galaxy would see the same relative motion of all but the closest galaxies away from her.
. A compact neutron star has a mass of kg (about 1.4 times the mass of the Sun) but a radius of only m (approximately 6.2 mi!). If a clock on the surface of this exotic star marks the passage of 1 h of time, how much time is observed to pass on an identical clock located a very large distance from the neutron star?
(a) Calculate The approximate age of the universe from the average value of the Hubble constant, To do this, calculate the time it would take to travel 1 Mly at a constant expansion rate of 20 km/s. (b) If deceleration is taken into account, would the actual age of the universe be greater or less than that found here? Explain.
The nearest neutron star (a collated star made primarily of neutrons) is about 3.00 1018 m away from Earth. Given that the Milky Way galaxy (Fig. P1.81) is roughly a disk of diameter 1021 m and thickness 1019 m, estimate the number of neutron stars in the Milky Way to the nearest order of magnitude. Figure P1.81
Distances to local galaxies are determined by measuring the brightness of stats, called Cepheid variables, that can be observed individually and that have absolute brightnesses at a standard distance that are well known. Explain how the measured brightness would vary with distance as compared with the absolute brightness.
The distance of a galaxy from our solar system is 10 Mpc. (a) What is the recessional velocity of the galaxy? (b) By what fraction is the starlight from this galaxy red shifted (that is, what is its z value)?
A galaxy is observed to recede from Earth with an approximate speed of 0.81c. Approximately how far d from Earth is this galaxy? Give an answer in units of megaparsecs (Mpc). d = ? Mpc How long ago t was the light that we see emitted by the galaxy? Give an answer in units of years. t = ? years
The kinetic energy of the ejecta from a supernova explosion is about 10^44 joules. Use the formula for kinetic energy to determine the typical speed at which matter is ejected from a supernova with a mass of 10 Msun. Compare that speed with the Sunʹs orbital speed around our galaxy. Based on your comparison, do you think the galaxyʹs gravity would be strong enough to retain the supernova debris if there were no interstellar medium to slow it down? Explain.
Suppose that the universe were full of spherical objects, each of mass m and radius r . If the objects were distributed uniformly throughout the universe, what number density (#/m3) of spherical objects would be required to make the density equal to the critical density of our Universe? Values: m = 4 kg r = 0.0407 m Answer must be in scientific notation and include zero decimal places (1 sig fig --- e.g., 1234 should be written as 1*10^3)
Another commonly calculated velocity in galactic dynamics is the escape velocity vesc, that is the minimum velocity a star must have in order to escape the gravitational field of the galaxy. (a) Starting from the work required to move a body over a distance dr against f show that the escape velocity from a point mass galaxy is vsc = 2GM/r where r is your initial distance. (b) Since we know galaxies aren't actually point-masses, also show that vesc from r for a galaxy with a p(r) xr¯² density profile is vese that R is a cutoff radius at which the mass density is zero. = 2v(1+ ln(R/r)). Here you must assume (c) The largest velocity measured for any star in the solar neighbourhood, at r=8 kpc, is 440 km/s. Assuming that this star is still bound to the galaxy, find the lower limit (in kiloparsecs), to the cutoff radius R and a lower limit (in solar units) to the mass of the galaxy. Note the solar rotation velocity is 220 km/s.
Another commonly calculated velocity in galactic dynamics is the escape velocity vesc, that is the minimum velocity a star must have in order to escape the gravitational field of the galaxy. (a) Starting from the work required to move a body over a distance dr against f show that the escape velocity from a point mass galaxy is vse = 2GM/r where r is your initial distance. (b) Since we know galaxies aren't actually point-masses, also show that vesc from r for a galaxy with a p(r) x r-² density profile is vse = 2v²(1+ ln(R/r)). Here you must assume that R is a cutoff radius at which the mass density is zero. (c) The largest velocity measured for any star in the solar neighbourhood, at r=8 kpc, is 440 km/s. Assuming that this star is still bound to the galaxy, find the lower limit (in kiloparsecs), to the cutoff radius R and a lower limit (in solar units) to the mass of the galaxy. Note the solar rotation velocity is 220 km/s.

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