ASTR 207 – Ch. 26: Cosmology – Flashcards
Unlock all answers in this set
Unlock answersquestion
Cosmology
answer
The study of the structure and evolution of the entire universe.
question
Sloan Great Wall
answer
The largest known structure in the universe, stretches 300 mpc. There is nothing larger than this.
question
Pencil Beam Survey
answer
A narrow field of view, only a few small patches in the sky, that studies extremely faint, hence far away, galaxies. Clusters and walls show up as 'spikes' in a graph.
question
Galaxy Survey
answer
A redshift survey that explores the large-scale distribution of galaxies in space.
question
The Cosmological Principle
answer
Homogenous - the universe is the same everywhere on scales greater than a few 100 mpc Isotropic - the universe is the same in all directions.
question
Implications of the Cosmological Principle
answer
- Implies that because the universe is the same everywhere, there is no edge to the universe (homogeny) - Implies that there is no center, because that would mean that the universe would not be the same in all directions from any non-central point.
question
In what sense, and on what scale, is the universe homogeneous and isotropic?
answer
On very large scales - more than 300 mpc - the distribution of galaxies seems to be roughly the same in all directions.
question
Olber's paradox
answer
A thought experiment suggesting that if the universe were homogenous, infinite, and unchanging, the entire night sky would be as bright as the surface of the sun.
question
Recession Velocity
answer
Equal to Hubble's Constant (70 km/s/mpc)x distance
question
Primeval Fireball
answer
Hot, dense state at very early times, just after the Big Bang. According to Hubble's Law, 14 billion years ago all the galaxies in the universe lay on top of each other. Then, the universe began to expand at a rapid rate, and its density and temperature fell rapidly as the volume increased.
question
The Big Bang
answer
Event that cosmologists consider the beginning of the universe, in which all matter and radiation in the entire universe came into being.
question
The Expansion of the Universe
answer
- Hubble's law describes the expansion of the universe. - The universe expands with space homogenously, it does not expand into the empty spaces between galaxies, for example. This does not mean, though, that the galaxies themselves expand.
question
Cosmological Redshift
answer
As a photon moves in space, its wavelength is influenced by the expansion of the universe. As the universe expands, photons of radiation are stretched in wavelength, giving rise to the cosmological redshift. Photons with a larger redshift are OLDER because the universe was SMALLER when they were emitted.
question
Why does Hubble's law imply a Big Bang?
answer
Because tracing the motion backwards in time, it implies that all galaxies, and in fact everything in the entire universe, were located at a single point at the same instant in the past.
question
Escape Speed
answer
The speed necessary for one object to escape the gravitational pull of another. Anything that moves away from a gravitating body with more than escape speed will never return.
question
Two possibilities for the Universe
answer
Either the universe will continue expanding (if it is low density) or the universe will stop expanding and collapse (if it is high density)
question
Critical density
answer
9x 10 to the power of -27 kg/m3 The density corresponding to a universe in which gravity acting alone would be sufficient to halt the present expansion.
question
Two Futures - High Density
answer
HIGH DENSITY: - If the cosmos emerged from the Big Bang with sufficiently high density, then it contains enough matter to halt its own expansion and the recession of galaxies will stop. Red shifts will turn to blue shifts as the universe begins collapsing inward and stars and planets and galaxies will collide with increasing frequency and violence as space diminishes. The entire universe will shrink towards a super dense, super hot singularity, much like the one from which it originated, the "big crunch"
question
Two Futures - Low Density
answer
Gravity of a low density universe will be too weak to half the present expansion. The universe will expand forever, the galaxies continually receding, their radiation steadily weakening with increasing distance. In time, we will see no galaxies in the sky beyond the Local Group. Eventually, the Local Group will peter out as their fuel supply is consumed. A "cold death" will happen whereby all radiation, matter, and life are eventually destined to freeze. This would take about a trillion years to happen.
question
What are the 2 basic properties for the future expansion of the universe?
answer
The universe can expand forever, in which case we die a cold death in which all activity gradually fades away, or the expansion can stop and the universe will recollapse to a fiery Big Crunch.
question
Relativity
answer
The presence of matter or energy causes a warping, or curvature of spacetime and the curved trajectories of free falling particles within warped spacetime are what Newton thought of as orbits under the influence of gravity. The amount of warping depends on the amount of matter present.
question
What actually contributes to the curvature of space?
answer
Both matter and energy must be taken into account, with energy properly converted into matter units by division by the square of the speed of light. (3x10 to the power of 8m/s)squared = 1.1x10 to the power of minus 17 kg - Density of the universe then includes not only atoms and molecules, but also invisible dark matter and everything that carries energy.
question
Cosmic density parameter
answer
The ratio of the universe's actual density to the critical value. Critical density - universe with density equal to critical density Critical density of less than 1 - universe with low density Critical density of more than 1 - universe with high density
question
Closed universe
answer
Geometry that the universe as a whole would have if the density of matter is ABOVE the critical value at high density. A closed universe is finite in extent and has no edge, like the surface of a sphere. It has enough mass to stop the present expansion and will eventually collapse.
question
Positive Curvature
answer
The surface of a sphere curves in the 'same direction' from any given point in a closed or high density universe.
question
Open Universe
answer
Geometry that the universe would have if the density of matter were less than the critical value. In an open universe, there is not enough matter to halt expansion of the universe. The universe is infinite.
question
Negative Curvature
answer
The surface of the universe curves like a saddle in a low density or open universe (infinite)
question
Critical Universe
answer
Universe in which the density of matter is equal to the critical density. The universe in infinite in extent and has zero curvature. Expansion will continue forever but will approach an expansion speed of zero.
question
How is the curvature of space related to the density of the universe?
answer
A low density universe has negative curvature, a critical density universe is spatially flat (Euclidean), and a high-density universe has positive curvature (and is finite in extent)
question
Density of the universe
answer
It seems that, when averaged over the entire universe, even taking into account dark matter, the density of the universe isn't much more than 30% critical density. Therefore, the universe is low density and will continue to expand forever.
question
Decelerating Universe
answer
Because expansion rate is decreasing, objects at great distances, that is, objects that emitted their radiation long ago, should appear to be receding FASTER.
question
Accelerating Universe
answer
Because expansion is increasing, far away objects recede less than Hubble's law would predict.
question
Dark Energy
answer
Generic name given to the unknown cosmic force field thought to be responsible for the observed acceleration of the Hubble expansion. The expansion of the universe is opposed by the attractive force of gravity and sped up by the repulsion due to dark energy. As the universe expands, the gravitational force weakens, whereas the force due to dark energy increases.
question
Cosmological Constant
answer
Quantity originally introduced by Einstein into general relativity to make his equations describe a static universe. Now one of several candidates for the repulsive 'dark energy' force responsible for the observed cosmic acceleration.
question
Why do astronomers think the universe will expand forever?
answer
There doesn't seem to be enough matter to halt the collapse, and in addition, the observed cosmic acceleration suggests the existence of a large-scale repulsive force in the cosmos that also opposes recollapse.
question
Geometry of the Universe
answer
As best we can tell, the universe on the largest scales is geometrically flat.
question
Two forces involved in the expansion of the universe.
answer
GRAVITY tends to slow expansion while DARK MATTER tends to accelerate it.
question
Cosmic Age
answer
Both the expansion and the age of the cosmos are governed by the competition between gravity and dark energy. According to current observations, dark energy seems destined to win. The age of a universe without dark energy is always less than 1/Hubbles Constant (70km/s/mpc) and DECREASES for larger values of present-day density. --> The existence of a repulsive cosmological constant INCREASES the age of the cosmos. The age of the universe appears to be 1/Hubble's Constant (70 km/s/mpc) - Our current best guess of the age of the universe is 14 billion years***
question
Why have astronomers concluded that dark energy is the major constituent of the universe?
answer
The observed acceleration of the universe implies that some nongravitational force must be at work. Dark energy is our current best theory of the cause of that force. In addition, galactic and cosmological observations indicate that the universe is spatially flat, and hence has critical density, but that the density of matter (mostly dark) cannot account for the cosmic acceleration is in good agreement with the required extra density (about 70% of critical value)
question
Cosmic microwave background
answer
The almost perfectly isotropic radio signal that is the electromagnetic remnant of the Big Bang,
question
When was the cosmic microwave background formed?
answer
At the time of the Big Bang. It is the electromagnetic remnant of the primeval fireball.
question
What evidence do we have that there is no structure in the universe on very large scales? How large is very large?
answer
Based on galaxy surveys and pencil beam surveys, we know that the universe is both homogeneous and isotropic. 300 mpc is the larger scale, there is no known structure larger.
question
What is the cosmological principle?
answer
The universe is homogeneous and isotropic, that is it is the same everywhere on large scales.
question
What is Olbers's paradox? How is it resolved?
answer
Olbers's paradox is the obvious contradiction that the entire night sky should be extremely luminous due to the amount of stars in the sky, and the fact that the sky is actually dark at night. Olbers' paradox is resolved in that, as Olbers assumed, the universe is not infinite and has evolved over time, contrary to his beliefs.
question
Explain how an accurate measure of Hubble's constant can lead to an estimate of the age of the universe.
answer
Age is inversely proportional to Hubble's constant. Because Hubble's Constant (70km/s/mpc) is indicative of the rate at which the universe is expanding, we simply have to divide by one.
question
We appear to be at the centre of the Hubble Flow. Why doesn't this observation violate the cosmological principle?
answer
Every other place in the universe also appears to be at the centre.
question
Why isn't it correct to say that the expansion of the universe involves galaxies flying outward into empty space?
answer
Because the only way we can have Hubble's law hold and retain the cosmological principle is to realize that the Big Bang involved the entire universe, not just the matter and radiation within it, but the universe itself, which is expanding.
question
Where did the Big Bang occur?
answer
The Big Bang did not happen at any particular location in space, because space itself was being compressed to a point at that instant - the Big Bang happened everywhere at once.
question
How does the cosmological redshift relate to the expansion of the universe?
answer
It occurs as a photon's wavelength is 'stretched' by cosmic expansion. The extent of the observed redshift is a direct measure of the expansion of the universe since the photon was emitted.
question
What properties of the universe determine whether it will or will not expand forever?
answer
The DENSITY and MASS of the universe will determine this. If the universe is OVER critical density (9x10 to the power of -27kg/m3) then it will continue to expand until everything simply disappears and the Milky Way becomes cold and dark.
question
What will become the ultimate fate of the universe if it does not expand forever?
answer
Expansion will stop, but gravity will continue and the universe will contract. Nearby galaxies will blueshift (move towards us) and the density and temperature of the universe will rise as it starts to collapse in on itself. Stars and galaxies will collide violently as the available space diminishes and the universe shrinks towards a super hot and super dense singularity much like the one from which we originated, 'the big crunch'
question
Is there enough luminous matter to halt the current cosmic expansion?
answer
No. Even with dark matter considered the universe is approximately only at 30% critical density.
question
Is there enough dark matter to halt the current cosmic expansion?
answer
No. Even with dark matter considered the universe is approximately only at 30% critical density.
question
What do observations of distant supernovae tell us about the expansion of the universe?
answer
They allow astronomers to measure changes in the rate of expansion of the universe. In a decelerating universe, supernovae would show a greater increase in redshift than predicted by Hubble's law. In an accelerating universe (such that we've observed), there is more of a decrease in redshift than predicted by Hubble's law.
question
What is the cosmological constant and what does it tell us about the universe?
answer
It is one leading dark-energy candidate, an additional 'vacuum energy' force associated with empty space and effective only on very large scales. It tells us that the universe is constantly expanding as it has an overall repulsive effect.
question
Why are the measurements of globular cluster ages important to cosmology?
answer
These ancient star clusters are thought to have formed at around the same time as our galaxy, so they date the time of galaxy formation.
question
What is the significance of the cosmic microwave background?
answer
It is an isotropic blackbody radiation field that fills the entire universe. Its present temperature is 3K. The existence of the microwave background is direct evidence that the universe expanded from a hot, dense, state.
question
Why does the temperature of the microwave background fall as the universe expands?
answer
This is a consequence of Earth's motion through space. We are moving with respect to that frame of reference, so the radiation in front of us is blueshifted white behind us it is redshifted. In reality, the microwave background temperature is isotropic.
question
How can we measure Earth's motion with respect to the universe?
answer
The only thing we can ever hope to do in an expanding universe is to determine our velocity (speed and direction) with respect to ever larger collections of matter, and larger scales. The Earth moves at about 30 km/sec in an orbit centered on the Sun. The sun looks like a stationary object locally, but we know it is moving around the center of the galaxy with a speed of 220 km/sec. For many kinds of astronomical study, the center of the Milky Way is 'at rest' and stars and gas clouds circulate around it. For other studies, we have to consider the motion of our galaxy relative to the center of mass of our local cluster of galaxies, or the motion of our local cluster relative to more distant clusters. The speeds and directions of all these motions, at any given time, have to be vectorially added.
question
If observations made from the middle of a large city are isotropic then
answer
There are tall buildings in every direction
question
The cosmological principle would be invalidated if we found that
answer
The observed structure of the universe depends on the direction in which we look.
question
When we use Hubble's law to estimate the age of the universe the answer we get
answer
is the same for all galaxies
question
Olbers's paradox is resolved by
answer
the finite age of the universe
question
The galactic distances use to measure the acceleration of the universe are determine by observations of
answer
exploding white dwarfs
question
The observed acceleration of the universe means that
answer
the amount of dark energy exceeds the total mass-energy of matter in the universe.
question
On the basis of our current best estimate of the present mass density of the universe, astronomers think that
answer
The universe is infinite in extent and will expand forever.
question
The age of the universe is estimated to be
answer
greater than the age of the Milky Way
question
The cosmic background radiation is observed to come from
answer
All directions equally
