Hubble's law
Definition: Hubble's law states that a galaxy's recession velocity v is proportional to its distance d: v = H0 d, where H0 (the Hubble constant) sets the current expansion rate of the universe.
Why it matters here: Hubble’s law converts a redshift-distance relationship into an expansion rate and, from that, an approximate age for the universe, closing the course’s arc from individual stars to the observational foundations of modern cosmology.
Relationship: Hubble’s law describes the expansion of space itself between galaxies; there is no center of the expansion, and every observer in the universe sees the same pattern of galaxies receding in proportion to distance.
Boundary or common confusion: Interpreting galaxies’ recession as galaxies "moving through" space away from us, the way an explosion scatters debris from a central point. Hubble’s law describes the expansion of space itself between galaxies; there is no center of the expansion, and every observer in the universe sees the same pattern of galaxies receding in proportion to distance.
Equation connection: \( v = H_0\,d, \qquad t_{\rm Hubble} = \dfrac{1}{H_0} \)
Hubble constant
Definition: Hubble's law states that a galaxy's recession velocity v is proportional to its distance d: v = H0 d, where H0 (the Hubble constant) sets the current expansion rate of the universe.
Why it matters here: Hubble’s law converts a redshift-distance relationship into an expansion rate and, from that, an approximate age for the universe, closing the course’s arc from individual stars to the observational foundations of modern cosmology.
Relationship: Because H0 has units of inverse time, its reciprocal (the Hubble time, 1/H0) gives an order-of-magnitude estimate for the age of the universe, exact only for a universe that has expanded at a perfectly constant rate (which ours has not, given the accelerating expansion of Lecture 13).
Boundary or common confusion: Interpreting galaxies’ recession as galaxies "moving through" space away from us, the way an explosion scatters debris from a central point. Hubble’s law describes the expansion of space itself between galaxies; there is no center of the expansion, and every observer in the universe sees the same pattern of galaxies receding in proportion to distance.
Equation connection: \( v = H_0\,d, \qquad t_{\rm Hubble} = \dfrac{1}{H_0} \)
Hubble time
Definition: Because H0 has units of inverse time, its reciprocal (the Hubble time, 1/H0) gives an order-of-magnitude estimate for the age of the universe, exact only for a universe that has expanded at a perfectly constant rate (which ours has not, given the accelerating expansion of Lecture 13).
Why it matters here: Hubble’s law converts a redshift-distance relationship into an expansion rate and, from that, an approximate age for the universe, closing the course’s arc from individual stars to the observational foundations of modern cosmology.
Relationship: Hubble time: 1/H0 = 1 / (71.5 km/s/Mpc), converting units (1 Mpc = 3.086e+19 km) gives t_Hubble ≈ 13.7 billion years — remarkably close to the independently measured age of the universe from the cosmic microwave background (about 13.8 billion years), especially given this estimate assumes a constant expansion rate rather than the universe’s actual expansion history.
Boundary or common confusion: Interpreting galaxies’ recession as galaxies "moving through" space away from us, the way an explosion scatters debris from a central point. Hubble’s law describes the expansion of space itself between galaxies; there is no center of the expansion, and every observer in the universe sees the same pattern of galaxies receding in proportion to distance.
Equation connection: \( v = H_0\,d, \qquad t_{\rm Hubble} = \dfrac{1}{H_0} \)
redshift
Definition: The cosmic microwave background (a near-perfect 2.7 K blackbody radiation field filling the entire sky, discovered accidentally by Penzias and Wilson in 1965) is the cooled, redshifted afterglow of the hot, dense early universe, predicted by the Big Bang model decades before its detection.
Why it matters here: Hubble’s law converts a redshift-distance relationship into an expansion rate and, from that, an approximate age for the universe, closing the course’s arc from individual stars to the observational foundations of modern cosmology.
Relationship: State Hubble’s law and use redshift-distance data to estimate the Hubble constant.
Boundary or common confusion: Interpreting galaxies’ recession as galaxies "moving through" space away from us, the way an explosion scatters debris from a central point. Hubble’s law describes the expansion of space itself between galaxies; there is no center of the expansion, and every observer in the universe sees the same pattern of galaxies receding in proportion to distance.
cosmic microwave background
Definition: The cosmic microwave background (a near-perfect 2.7 K blackbody radiation field filling the entire sky, discovered accidentally by Penzias and Wilson in 1965) is the cooled, redshifted afterglow of the hot, dense early universe, predicted by the Big Bang model decades before its detection.
Why it matters here: Hubble’s law converts a redshift-distance relationship into an expansion rate and, from that, an approximate age for the universe, closing the course’s arc from individual stars to the observational foundations of modern cosmology.
Relationship: Hubble time: 1/H0 = 1 / (71.5 km/s/Mpc), converting units (1 Mpc = 3.086e+19 km) gives t_Hubble ≈ 13.7 billion years — remarkably close to the independently measured age of the universe from the cosmic microwave background (about 13.8 billion years), especially given this estimate assumes a constant expansion rate rather than the universe’s actual expansion history.
Boundary or common confusion: Interpreting galaxies’ recession as galaxies "moving through" space away from us, the way an explosion scatters debris from a central point. Hubble’s law describes the expansion of space itself between galaxies; there is no center of the expansion, and every observer in the universe sees the same pattern of galaxies receding in proportion to distance.
Big Bang nucleosynthesis
Definition: Independent observational pillars — the expansion itself (Hubble’s law), the cosmic microwave background, and the observed primordial abundances of hydrogen, helium, and lithium (Big Bang nucleosynthesis) — together support the Big Bang model of a universe that began in a hot, dense state roughly 13.8 billion years ago.
Why it matters here: Hubble’s law converts a redshift-distance relationship into an expansion rate and, from that, an approximate age for the universe, closing the course’s arc from individual stars to the observational foundations of modern cosmology.
Relationship: The cosmic microwave background (a near-perfect 2.7 K blackbody radiation field filling the entire sky, discovered accidentally by Penzias and Wilson in 1965) is the cooled, redshifted afterglow of the hot, dense early universe, predicted by the Big Bang model decades before its detection.
Boundary or common confusion: Interpreting galaxies’ recession as galaxies "moving through" space away from us, the way an explosion scatters debris from a central point. Hubble’s law describes the expansion of space itself between galaxies; there is no center of the expansion, and every observer in the universe sees the same pattern of galaxies receding in proportion to distance.