21st Century Astronomy
6th Edition
ISBN: 9780393428063
Author: Kay
Publisher: NORTON
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Chapter 22.1, Problem 22.1CYU
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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 = 12×106
Assume 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_0
What would be the critical density of matter in the universe if the value of the Hubble constant were (a) 40 km/sec/ Mpc? (b) 90 km/sec/Mpc?
Chapter 22 Solutions
21st Century Astronomy
Ch. 22.1 - Prob. 22.1CYUCh. 22.2 - Prob. 22.2CYUCh. 22.3 - Prob. 22.3CYUCh. 22.4 - Prob. 22.4CYUCh. 22.5 - Prob. 22.5CYUCh. 22 - Prob. 1QPCh. 22 - Prob. 2QPCh. 22 - Prob. 3QPCh. 22 - Prob. 4QPCh. 22 - Prob. 5QP
Ch. 22 - Prob. 6QPCh. 22 - Prob. 7QPCh. 22 - Prob. 8QPCh. 22 - Prob. 9QPCh. 22 - Prob. 10QPCh. 22 - Prob. 11QPCh. 22 - Prob. 12QPCh. 22 - Prob. 13QPCh. 22 - Prob. 14QPCh. 22 - Prob. 15QPCh. 22 - Prob. 16QPCh. 22 - Prob. 17QPCh. 22 - Prob. 18QPCh. 22 - Prob. 19QPCh. 22 - Prob. 20QPCh. 22 - Prob. 21QPCh. 22 - Prob. 22QPCh. 22 - Prob. 24QPCh. 22 - Prob. 28QPCh. 22 - Prob. 29QPCh. 22 - Prob. 31QPCh. 22 - Prob. 32QPCh. 22 - Prob. 33QPCh. 22 - Prob. 34QPCh. 22 - Prob. 35QPCh. 22 - Prob. 36QPCh. 22 - Prob. 37QPCh. 22 - Prob. 38QPCh. 22 - Prob. 39QPCh. 22 - Prob. 40QPCh. 22 - Prob. 41QPCh. 22 - Prob. 42QPCh. 22 - Prob. 43QPCh. 22 - Prob. 44QPCh. 22 - Prob. 45QP
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- Why couldnt atomic nuclei exist when the Universe was less than about 2 minutes old?arrow_forwardThe matter density in the Universe today is ?m=2.7×10−27kgm−3. What would be the value of the density parameter, Ω0, if the Hubble constant had the value H0 = 38 km/s/Mpc?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).arrow_forward
- By what factor would the critical density of the universe today change if the Hubble constant today measured at 5 times greater than what we previously thought?arrow_forwardAssume a flat Friedmann-Robertson-Walker universe.Calculate the redshift z at which the temperature of the CMB was T = 10 T0, where T0 is thetemperature of the CMB today. Assuming the universe was matter dominated throughout itsevolution, calculate the time at which T = 10 T0 in Gigayears.arrow_forwardUsing Ho = 20.8 km/s/Mly and assuming the expansion rate has been constant, find out how long the universe has been expanding.arrow_forward
- If the universe had a Hubble Constant (Ho) of 100km/s/Mpc and a critical density of 1. The universe will collapse on itself and come to an end collapse on itself but regenerate with a bounce expand and the speed of expansion will increase with time expand and the speed of expansion will decrease with timearrow_forwardThe background radiation has an average temperature of 2.7 K, using Wien’s Law, the current background radiation in the microwave region, the peak wavelength is 1mm. In the past, when the cosmic background radiation had a peak wavelength of 51.1 µm, calculate the relative size of the universe compared to the current size of the universe, that is, the universe was how much smaller by a factor of what? Round to TWO places past the decimalarrow_forwardThe 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.arrow_forward
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