College Physics
OER 2016 Edition
ISBN: 9781947172173
Author: OpenStax
Publisher: OpenStax College
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Textbook Question
Chapter 34, Problem 28PE
The critical mass density needed to just halt the expansion of the universe is approximately 
(a) Convert this to 
(b) Find the number of neutrinos per cubic meter needed to close the universe if their average mass is
and they have negligible kinetic energies.
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Students have asked these similar questions
The critical mass density needed to just halt the expansion of the universe is approximately 10-26 kg/m3. Find the number of neutrinos per cubic meterneeded to close the universe if their average mass is 7 eV/c2 and they have negligible kinetic energies.
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)
I asked the following question and was given the attached solution:
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)
I don't follow the work and I got the wrong answer, so please help and show your work as I do not follow along easily
thanks
Chapter 34 Solutions
College Physics
Ch. 34 - Explain why it only appears that we are at the...Ch. 34 - If there is no observable edge to the universe,...Ch. 34 - If the universe is infinite, does it have a...Ch. 34 - Another known cause of red shift in light is the...Ch. 34 - If some unknown cause of red shiftsuch as light...Ch. 34 - Olbers’s paradox poses an interesting question: If...Ch. 34 - If the cosmic microwave background radiation...Ch. 34 - The decay of one type of Kmeson is cited as...Ch. 34 - Distances to local galaxies are determined by...Ch. 34 - Distances to very remote galaxies are estimated...
Ch. 34 - If the smallest meaningful time interval is...Ch. 34 - Quantum gravity, if developed, would be an...Ch. 34 - Does observed gravitational lensing correspond to...Ch. 34 - Suppose you measure the red shifts of all the...Ch. 34 - What are gravitational waves, and have they yet...Ch. 34 - Is the event horizon of a black hole the actual...Ch. 34 - Suppose black holes radiate their mass away and...Ch. 34 - Discuss the possibility that star velocities at...Ch. 34 - How does relativistic time dilation prohibit...Ch. 34 - If neutrino oscillations do occur, will they...Ch. 34 - Lacking direct evidence of WIMPs as dark matter,...Ch. 34 - Must a complex system be adaptive to be of...Ch. 34 - State a necessary condition for a System to be...Ch. 34 - What is critical temperature Tc? Do all materials...Ch. 34 - Explain how good thermal contact with liquid...Ch. 34 - Not only is liquid nitrogen a cheaper coolant than...Ch. 34 - For experimental evidence particularly of...Ch. 34 - Discuss whether you think there are limits to what...Ch. 34 - Find the approximate mass of the luminous matter...Ch. 34 - Find the approximate mass of the dark and luminous...Ch. 34 - (a) Estimate the mass of the luminous matter in...Ch. 34 - If a galaxy is 500 Mly away from us, how fast do...Ch. 34 - On average, how far away are galaxies mat are...Ch. 34 - Our solar system orbits the center of the Milky...Ch. 34 - (a) What is the approximate speed relative to us...Ch. 34 - (a) Calculate The approximate age of the universe...Ch. 34 - Assuming a circular orbit for the Sun about the...Ch. 34 - (a) What is the approximate force of gravity on a...Ch. 34 - Andromeda galaxy is the closest large galaxy and...Ch. 34 - (a) A particle and its antiparticle are at rest...Ch. 34 - The average particle energy needed to observe...Ch. 34 - The peak intensity of the CMBR occurs at a...Ch. 34 - (a) What Hubble constant corresponds to an...Ch. 34 - Show that the velocity of a star orbiting its...Ch. 34 - The core of a star collapses during a supernova,...Ch. 34 - Using data from the previous problem, find the...Ch. 34 - Distances to the nearest stars (up to 500 by away)...Ch. 34 - (a) Use the Heisenberg uncertainty principle to...Ch. 34 - Construct Your Own Problem Consider a star moving...Ch. 34 - What is the Schwarzschild radius of a blank hole...Ch. 34 - Black holes with masses smaller than muse formed...Ch. 34 - Supermassive black holes are thought to exist at...Ch. 34 - Construct Your Own Problem Consider a supermassive...Ch. 34 - The characteristic length of entities in...Ch. 34 - If the dark matter in the Milky Way were composed...Ch. 34 - The critical mass density needed to just halt the...Ch. 34 - Assume the average density of the universe is 0.1...Ch. 34 - To get an idea of how empty deep spam is on the...Ch. 34 - A section of superconducting wire carries a...
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- The energy density ϵ in radiation is related to its temperature by ϵ=αT4. Compute the temperature when the Universe was 0.1 second old, using the Friedmann equation and its radiation-dominated solution a(t)∝t1/2.arrow_forwardIf the current mass of the universe consists of 75% protons and 25% 4He, what is the current ratio of protons to neutrons?arrow_forwardThe critical mass density needed to just halt the expansion of the universe is approximately 10-26 kg/m3. Convert this to eV/c2.m3 .arrow_forward
- Consider a universe where Big Bang nucleosynthesis produced significantly more 4He than 1H. Estimate the observed redshift, z, of the Cosmic Radiation Background (CMB) by an observer that observes the CMB to have a blackbody temperature of 2.715 K. Assume this universe has Ob = 0. 0486, QDM = O. 2588 and QA = 0. 6911arrow_forwardThe mass of the neutrino plays an important role in the universe. Suppose the mass of two neutrinos in the universe is 4.8×10-³5 kg and the current Hubble’s constant is 72 km/s/Mpc. The critical density of the universe is five times the average density of the universe. Estimate the number of neutrons present per cubic meter in the universe. (a) 2.1×10⁹ (b) 4.1×10² (c) 1.1x10° (d) 8.1×107arrow_forwardAssume the average density of the Universe is equal to the critical density. (a) Prove that the age of the Universe is given by 2/(3H). (b) Calculate 2/(3H) and express it in years.arrow_forward
- If the average density of the Universe is small compared with the critical density, the expansion of the Universe described by Hubble's law proceeds with speeds that are nearly constant over time. Calculate t since the big bang, assuming H = 22.0 km/s/Mly.arrow_forwardTo get an idea of how empty deep space is on the average, perform the following calculations: (a) Find the volume our Sun would occupy if it had an average density equal to the critical density of 10-26 kg / m3 thought necessary to halt the expansion of the universe. (b) Find the radius of a sphere of this volume in light years. (c) What would this radius be if the density were that of luminous matter, which is approximately 5% that of the critical density? (d) Compare the radius found in part (c) with the 4-ly average separation of stars in the arms of the Milky Way.arrow_forwardSuppose the energy density of the cosmological constant is equal to the present critical density En = Ec.0 = 4870MeVm-3. What is the total energy of the cosmological constant within a sphere 1AU in radius?What is the rest energy of the Sun (Eo = Moc²)? Comparing these two numbers, do you expect the cosmological constant to have a significant effect on the motion of planets within the solar system?arrow_forward
- Explain what is meant by the term cosmological redshift, and contrast it with the Doppler shift that is observed when a luminous object recedes from an observer. From the defi- nition of redshift in terms of the observed and emitted wavelengths of photons, obtain an expression that relates redshift to the expansion factor of the Universe. Hence calculate the expansion factor of the Universe while a photon travels from a galaxy with redshift z = 1.arrow_forwardThe visible section of the Universe is a sphere centered on the bridge of your nose, with radius 13.7 billion light-years. (a) Explain why the visible Universe is getting larger, with its radius increasing by one light-year in every year. (b) Find the rate at which the volume of the visible section of the Universe is increasing.arrow_forwardPlease answer within 90 minutes.arrow_forward
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