ASTRON C10 FINAL Exam Practice Test Actual
2025/2026 Questions And 100% Correct
Answers
cosmology - correct answer-
the study of the structure and evolution of the universe as a whole, how big, how old, what overall sha
pe, what fate
olber's paradox - correct answer-
why is the sky dark at night? sunset? if the universe is infinite in size and age and is uniformly filled
with stars (on average), then every line of sight should intersect a star so the sky should be very bright
all the time
surface brightness of stars - correct answer-
distant stars look dim (inverse square law), distant stars have small angular area, surface brightness = b
rightness per unit angular area, brightness spread over angular area (surface b independent of distance)
possible solutions to Olber's paradox - correct answer-
the universe has a finite size, the universe is infinitely big but there are few or no stars far away, the u
niverse has a finite age in which light from most stars has not reached us...
other possibilities: dust (would glow though), some are interesting but can be ruled out or violate the C
opernican Principle (stars are lined up along radical spokes centered on us -
so there would be big spaced in between us in the middle - not true)
olber's paradox: solution - correct answer-option 3 is the main solution: universe has a finite age (10-
14 billion y.o.) and light from the most distant stars has not reached us so the sky is dark (universe get
s brighter over time but CMBRKlessens and dims (leftover of Big Bang - not just 1? multiverse) -
universe is not just stars), expansion of universe can help resolve paradox because light from very dist
ant galaxies will be redshifted so much we won't see it at optical wavelengths (small effect compared t
o age)
expansion of the universe, hubble lemaitre law (can be used to determine our distance from another clu
ster of galaxies outside local group) - correct answer-
Hnought = hubble's constant (hubble's parameter), does not imply that the speed of a given galaxy incr
eases with time. hubble's constant is a constant in space not in time (generally decreases with time, in f
act, like critical density), graph: velocity left vs distance, hnought = 71 +/-
3 (km/s)/Mpc (1Mpc = 3.26 * 10^6 ly), hnought = 2.3*10^-18 1/s
,what's expanding? - correct answer-
speed is proportional to distance, every bit of space between clusters of galaxies, not on smaller scales,
almost all galaxies are moving away from us so are we the center of the universe's expansion -
NO (no unique center in the universe, expanding uniformly and wraps around), infinite loaf of wrap-
around raisin bread expanding uniformly, infinite rubber band with knots in it, surface of a spherical ba
lloon in which center of expansion is at center of the balloon which is not part of this hypothetical uni
verse (in a physically inaccessible dimension)
expansion starts with the big bang - correct answer-
no center (every cluster sees the same thing), the density of the universe is decreasing, the universe is
expanding (space itself) -
RK(scale factor) is getting larger with time and is the distance between any two clusters of galaxies in
general, when R=0 density and temperature = infinity...singularity = HOT (big bang but , extrapolate b
ack in time), galaxies themselves don't expand (gravitationally bound, electromagnetic force)
If v=constant then d=vt but v=Hnought*d (Hubble's law)
d=(v)(1/Hnought)=v*Tnought - correct answer-
If v constant with time, 1/(Hnought)=Tnought=age of the universe
If Hnought=50 km/s/Mpc, Tnought=20Gyr
If Hnought=71 km/s/Mpc, Tnought=13.7Gyr (as long as things don't slow down expansion)
if the speed (v) of a given galaxy always decreases with time (because gravity is slowing down expans
ion of the universe) - correct answer-
then tnought<Tnought=1/Hnought covering less additional distance
more deceleration (slowing down) gives an even younger universe tnought (tnought<Tnought=1/Hnoug
ht) - correct answer-cover even less ground
if the speed v of a given galaxy always increases with time because something is accelerating the expa
nsion of the universe - correct answer-then tnought>Tnought=1/Hnought
more acceleration (speeding up) gives an even older universe tnought (tnought>Tnought=1/Hnought) -
correct answer-covers even more ground
determining hubble's constant Hnought - correct answer-
how? measure distances and recession velocities of relatively nearby galaxies, recession velocity easy:
from redshift, accurate distance is hard to measure
, measuring distances of galaxies - correct answer-
1) angular size (approximate), 2) apparent brightness (b) with equation (if we know L and measure b, t
hen calculate d).
some objects of "known" L: cepheids (brighter and dimmer, pulsate slower=moreL, not standard candle
s but standardizable), supergiants, globular clusters, novae, galaxies, Type 1A SN (brightest, billion sol
ar L, from WD close to chandra limit)
2 1000W lightbulbs with one 2x [(1/2)^2] as far away will appear 1/4 as bright (250W)
measuring and applying Hnought - correct answer-
obtain distances and redshifts of many galaxies at different distances to determine Hnought, then 1/Hno
ught is the max. possible age of the decelerating universe, also knowing Hnought one can deduce dista
nces of extremely distant galaxies/quasars/SN/etc., as the universe ages Tnought increases and Hnought
decreases (value of Hubble's constant decreases with time and is thought to be constant)
the cosmological principle (CP) - correct answer-
mathematically can study cosmic expansion with Einstein's general theory of relativity (theory of gravit
y, to the universe as a whole), assumptions: on the largest size scales the universe is homogenous (sam
e average density) and isotropic (looks the same in all directions): a UNIFORM universe, the uniformit
y of the universe is called the cosmological principle
a uniform universe - correct answer-
homogeneous (same average density), isotropic (looks the same in all directions), homogenous but not
isotropic (matter lined up) -
circle = observable part of the universe, homogeneous and isotropic over large distances, some deviati
ons of universe expansion as galaxies and clusters of galaxies pull on each other (so Andromeda and
MWG are approaching each other/contracting)
evidence for CP - correct answer-
evidence for CP/a uniform universe: the largest structures appear to be superclusters and voids, CMBR
is the same everywhere, note: the average density of the universe can and does change with time (decr
easing with time, not perfect CP)
if the speed of a given galaxy always increases with time (an accelerating universe, converse of slowin
g down where Hubble time is max.), then 1/Hnought is - correct answer-
the minimum possible age of the universe
2025/2026 Questions And 100% Correct
Answers
cosmology - correct answer-
the study of the structure and evolution of the universe as a whole, how big, how old, what overall sha
pe, what fate
olber's paradox - correct answer-
why is the sky dark at night? sunset? if the universe is infinite in size and age and is uniformly filled
with stars (on average), then every line of sight should intersect a star so the sky should be very bright
all the time
surface brightness of stars - correct answer-
distant stars look dim (inverse square law), distant stars have small angular area, surface brightness = b
rightness per unit angular area, brightness spread over angular area (surface b independent of distance)
possible solutions to Olber's paradox - correct answer-
the universe has a finite size, the universe is infinitely big but there are few or no stars far away, the u
niverse has a finite age in which light from most stars has not reached us...
other possibilities: dust (would glow though), some are interesting but can be ruled out or violate the C
opernican Principle (stars are lined up along radical spokes centered on us -
so there would be big spaced in between us in the middle - not true)
olber's paradox: solution - correct answer-option 3 is the main solution: universe has a finite age (10-
14 billion y.o.) and light from the most distant stars has not reached us so the sky is dark (universe get
s brighter over time but CMBRKlessens and dims (leftover of Big Bang - not just 1? multiverse) -
universe is not just stars), expansion of universe can help resolve paradox because light from very dist
ant galaxies will be redshifted so much we won't see it at optical wavelengths (small effect compared t
o age)
expansion of the universe, hubble lemaitre law (can be used to determine our distance from another clu
ster of galaxies outside local group) - correct answer-
Hnought = hubble's constant (hubble's parameter), does not imply that the speed of a given galaxy incr
eases with time. hubble's constant is a constant in space not in time (generally decreases with time, in f
act, like critical density), graph: velocity left vs distance, hnought = 71 +/-
3 (km/s)/Mpc (1Mpc = 3.26 * 10^6 ly), hnought = 2.3*10^-18 1/s
,what's expanding? - correct answer-
speed is proportional to distance, every bit of space between clusters of galaxies, not on smaller scales,
almost all galaxies are moving away from us so are we the center of the universe's expansion -
NO (no unique center in the universe, expanding uniformly and wraps around), infinite loaf of wrap-
around raisin bread expanding uniformly, infinite rubber band with knots in it, surface of a spherical ba
lloon in which center of expansion is at center of the balloon which is not part of this hypothetical uni
verse (in a physically inaccessible dimension)
expansion starts with the big bang - correct answer-
no center (every cluster sees the same thing), the density of the universe is decreasing, the universe is
expanding (space itself) -
RK(scale factor) is getting larger with time and is the distance between any two clusters of galaxies in
general, when R=0 density and temperature = infinity...singularity = HOT (big bang but , extrapolate b
ack in time), galaxies themselves don't expand (gravitationally bound, electromagnetic force)
If v=constant then d=vt but v=Hnought*d (Hubble's law)
d=(v)(1/Hnought)=v*Tnought - correct answer-
If v constant with time, 1/(Hnought)=Tnought=age of the universe
If Hnought=50 km/s/Mpc, Tnought=20Gyr
If Hnought=71 km/s/Mpc, Tnought=13.7Gyr (as long as things don't slow down expansion)
if the speed (v) of a given galaxy always decreases with time (because gravity is slowing down expans
ion of the universe) - correct answer-
then tnought<Tnought=1/Hnought covering less additional distance
more deceleration (slowing down) gives an even younger universe tnought (tnought<Tnought=1/Hnoug
ht) - correct answer-cover even less ground
if the speed v of a given galaxy always increases with time because something is accelerating the expa
nsion of the universe - correct answer-then tnought>Tnought=1/Hnought
more acceleration (speeding up) gives an even older universe tnought (tnought>Tnought=1/Hnought) -
correct answer-covers even more ground
determining hubble's constant Hnought - correct answer-
how? measure distances and recession velocities of relatively nearby galaxies, recession velocity easy:
from redshift, accurate distance is hard to measure
, measuring distances of galaxies - correct answer-
1) angular size (approximate), 2) apparent brightness (b) with equation (if we know L and measure b, t
hen calculate d).
some objects of "known" L: cepheids (brighter and dimmer, pulsate slower=moreL, not standard candle
s but standardizable), supergiants, globular clusters, novae, galaxies, Type 1A SN (brightest, billion sol
ar L, from WD close to chandra limit)
2 1000W lightbulbs with one 2x [(1/2)^2] as far away will appear 1/4 as bright (250W)
measuring and applying Hnought - correct answer-
obtain distances and redshifts of many galaxies at different distances to determine Hnought, then 1/Hno
ught is the max. possible age of the decelerating universe, also knowing Hnought one can deduce dista
nces of extremely distant galaxies/quasars/SN/etc., as the universe ages Tnought increases and Hnought
decreases (value of Hubble's constant decreases with time and is thought to be constant)
the cosmological principle (CP) - correct answer-
mathematically can study cosmic expansion with Einstein's general theory of relativity (theory of gravit
y, to the universe as a whole), assumptions: on the largest size scales the universe is homogenous (sam
e average density) and isotropic (looks the same in all directions): a UNIFORM universe, the uniformit
y of the universe is called the cosmological principle
a uniform universe - correct answer-
homogeneous (same average density), isotropic (looks the same in all directions), homogenous but not
isotropic (matter lined up) -
circle = observable part of the universe, homogeneous and isotropic over large distances, some deviati
ons of universe expansion as galaxies and clusters of galaxies pull on each other (so Andromeda and
MWG are approaching each other/contracting)
evidence for CP - correct answer-
evidence for CP/a uniform universe: the largest structures appear to be superclusters and voids, CMBR
is the same everywhere, note: the average density of the universe can and does change with time (decr
easing with time, not perfect CP)
if the speed of a given galaxy always increases with time (an accelerating universe, converse of slowin
g down where Hubble time is max.), then 1/Hnought is - correct answer-
the minimum possible age of the universe
