What must be known to estimate the distance to a galaxy with the Hubble law?
The correct answer and explanation is:
To estimate the distance to a galaxy using Hubble’s law, the two key pieces of information required are the galaxy’s redshift (or velocity) and the Hubble constant.
The redshift of a galaxy measures how much the light from that galaxy has been stretched as the universe expands. When a galaxy is moving away from us, the wavelengths of the light it emits shift toward the red end of the spectrum. This shift is often measured using the Doppler effect. The greater the redshift, the faster the galaxy is receding from us.
Hubble’s law relates the velocity of a galaxy (its recession velocity) to its distance from us through the equation: v=H0×dv = H_0 \times d
Where:
- v is the velocity of the galaxy, determined from its redshift.
- H₀ is the Hubble constant, which describes the rate of expansion of the universe. Its value is currently estimated to be about 70 km/s per megaparsec, though there is some uncertainty in its exact value due to varying methods of measurement.
- d is the distance to the galaxy, which is the quantity we aim to estimate.
To calculate the distance using this law, we first need to measure the redshift of the galaxy, which can be done by observing the light it emits. From the redshift, we can calculate the galaxy’s velocity. With this velocity and the known value of the Hubble constant, we can then solve for the distance.
Hubble’s law provides a simple and powerful method for estimating distances to galaxies, especially those far beyond our local group, where other methods like parallax or Cepheid variable stars are not applicable. However, it is important to note that this law assumes the universe is homogenous and isotropic on large scales, which may not always hold true in localized regions.