UNDERSTANDING THE UNIVERSE(LL)-W/CODE
3rd Edition
ISBN: 9780393869903
Author: PALEN
Publisher: NORTON
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Chapter 17, Problem 7QAP
To determine
Reason why voids do not contain dark matter.
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1. 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.
An astronomer in an imaginary Universe observes a small patch of the sky (0.005% of the total) with very deep observations and
finds one million galaxies. How many galaxies can they estimate to be present in the whole Universe?
10 billion
20 billion
50 billion
O 70 billion
We know that dark matter must exist in galaxies. If not, many of the stars in a galaxy would ...
a.Expand and go supernova due to the expansion of the Universe
b.Fly away from the galaxy because the stars’ velocities are so large, and the galaxy would not have enough gravity to hold onto them
c.Orbit the center of the galaxy normally
d.Spiral in toward the center of the galaxy because most of the galaxy’s mass would be toward the center
Chapter 17 Solutions
UNDERSTANDING THE UNIVERSE(LL)-W/CODE
Ch. 17.1 - Prob. 17.1CYUCh. 17.2 - Prob. 17.2CYUCh. 17.3 - Prob. 17.3CYUCh. 17.4 - Prob. 17.4CYUCh. 17.5 - Prob. 17.5CYUCh. 17 - Prob. 1QAPCh. 17 - Prob. 2QAPCh. 17 - Prob. 3QAPCh. 17 - Prob. 4QAPCh. 17 - Prob. 5QAP
Ch. 17 - Prob. 6QAPCh. 17 - Prob. 7QAPCh. 17 - Prob. 8QAPCh. 17 - Prob. 9QAPCh. 17 - Prob. 10QAPCh. 17 - Prob. 11QAPCh. 17 - Prob. 12QAPCh. 17 - Prob. 13QAPCh. 17 - Prob. 14QAPCh. 17 - Prob. 15QAPCh. 17 - Prob. 16QAPCh. 17 - Prob. 17QAPCh. 17 - Prob. 18QAPCh. 17 - Prob. 19QAPCh. 17 - Prob. 20QAPCh. 17 - Prob. 21QAPCh. 17 - Prob. 22QAPCh. 17 - Prob. 23QAPCh. 17 - Prob. 24QAPCh. 17 - Prob. 25QAPCh. 17 - Prob. 26QAPCh. 17 - Prob. 27QAPCh. 17 - Prob. 28QAPCh. 17 - Prob. 29QAPCh. 17 - Prob. 30QAPCh. 17 - Prob. 31QAPCh. 17 - Prob. 32QAPCh. 17 - Prob. 33QAPCh. 17 - Prob. 34QAPCh. 17 - Prob. 35QAPCh. 17 - Prob. 36QAPCh. 17 - Prob. 37QAPCh. 17 - Prob. 38QAPCh. 17 - Prob. 39QAPCh. 17 - Prob. 40QAPCh. 17 - Prob. 41QAPCh. 17 - Prob. 42QAPCh. 17 - Prob. 43QAPCh. 17 - Prob. 44QAPCh. 17 - Prob. 45QAP
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- How are distant (young) galaxies different from the galaxies that we see in the universe today?arrow_forwardWhat percentage of matter is ordinary matter? What percentage is dark matter? What makes up the rest of the Universes density?arrow_forwardA cluster of galaxies is observed to have a recessional velocity of 60,000 km/s. Find the distance to the cluster. (Assume a Hubble constant of 22 km/s per million light-years.)arrow_forward
- We can detect 21-cm emission from other galaxies as well as from our own Galaxy. However, 21-cm emission from our own Galaxy fills most of the sky, so we usually see both at once. How can we distinguish the extragalactic 21-cm emission from that arising in our own Galaxy? (Hint: Other galaxies are generally moving relative to the Milky Way.)arrow_forwardGalaxies are found in the “walls” of huge voids; very few galaxies are found in the voids themselves. The text says that the structure of filaments and voids has been present in the universe since shortly after the expansion began 13.8 billion years ago. In science, we always have to check to see whether some conclusion is contradicted by any other information we have. In this case, we can ask whether the voids would have filled up with galaxies in roughly 14 billion years. Observations show that in addition to the motion associated with the expansion of the universe, the galaxies in the walls of the voids are moving in random directions at typical speeds of 300 km/s. At least some of them will be moving into the voids. How far into the void will a galaxy move in 14 billion years? Is it a reasonable hypothesis that the voids have existed for 14 billion years?arrow_forwardSuppose the stars in an elliptical galaxy all formed within a few million years shortly after the universe began. Suppose these stars have a range of masses, just as the stars in our own galaxy do. How would the color of the elliptical change over the next several billion years? How would its luminosity change? Why?arrow_forward
- In galaxy clusters, most of the galaxies appear to be elliptical galaxies because They formed in-situ from low-angular-momentum clouds They aren't really elliptical, but they look that way because they are very distant When galaxies collect in clusters, the proximity makes them puffier Galaxies in clusters probably had many merger events in the pastarrow_forwardAn astronomer observed the motions of some galaxies. Based on his observations, he made the following statements. Which one of them is most likely to be false? Take Hubble's constant to be 67 km/s/Mpc. A. A galaxy observed to be moving away from us at a speed of 70 km/s is at a distance of about 1 Mpc from us. B. A galaxy observed to be moving away from us at a speed of 700 km/s is at a distance of about 10 Mpc from us. C. A galaxy observed to be moving away from us at a speed of 7000 km/s is at a distance of about 100 Mpc from us. D. A galaxy observed to be moving away from us at a speed of 70000 km/s is at a distance of about 1 Gpc from us. Is the answer D? Thank you!arrow_forwardA star at a distance of 50000 light years from the center of a galaxy has an orbital speed of 100 km/s around the galactic center. What is the total mass of the galaxy located at distances smaller than 50000 light years from the center? A. 7.6 ×1010 solar masses B. 4.2 ×1011 solar masses C. 3.5 ×1010 solar masses D. 1.4 ×1011 solar masses Is the answer C? M = (r x v^2) / G = 50000 x 9.46e15 x (100000^2) / 6.67e-11 / 2e30 (the Sun's mass) = 3.55e10 solar massesarrow_forward
- All Radio Galaxies have the following number of plasma jets and lobes a) 2 b) 5 c) 1 d) 4 e)7arrow_forwardWhat happens when galaxies collide? A. Star collisions will be rare but the shapes of the galaxies will be largely distorted. B. The shapes of the galaxies will be largely distorted and many of the stars of one galaxy will collide with stars of the other galaxy. C. The shapes of the galaxies will be distorted and many stars will collide with stars of the other galaxy, as well as with other stars in the same galaxy. D. Star collisions will be rare and the two galaxies will just pass through each other without any changes. Is the answer A? Thank you!arrow_forwardFigure 2 shows the "rotation curve" of NGC 2742. It plots the “radial velocity (V)" (how fast material is moving either toward or away from us) that is measured for objects at different distances (R = radius") from the center of the galaxy. The center of the galaxy is at 0 kpc (kiloparsecs) with a speed of 9 km/sec away from us. (These velocities have been corrected for the observed tilt of the galaxy and represent true orbital velocities of the stars and gas.) 200 100 U4779 -100 As you can see, one side of the galaxy is moving with a negative velocity (spinning toward us), while the other side has a positive velocity (spinning away from us). Using Newton's gravity equation, we will be able to determine the gravitational mass of the entire galaxy and how the mass varies versus distance from the galaxy's center. -200 -8 8 -4 Radius (kpc) Read the following text carefully and follow the instructions: Select five radii spaced evenly from 0-10 kpc across the galaxy. Your selections should…arrow_forward
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