@Magrathea_staff @LIGO Cosmology can be really tricky sometimes, but I can try to clear some things up. We measure luminosity distance from gravitational waves sources, which tells us how far away an object would be based on the brightness (or luminosity) of the detected signal.
@Magrathea_staff @LIGO But luminosity distance can be a bit confusing when talking about really far away sources. We can't apply the usual d=vt here to convert the distance to the time in took the gravitational waves to travel.
@Magrathea_staff @LIGO If we converted the distance of 5.2Gpc to a time, assuming the gravitational waves travel at the speed of light, we'd estimate that the source was 17 billion light years away. That's older than the age of the universe, and definitely can't be right. Instead, we need cosmology.
@Magrathea_staff @LIGO What we're really interested in is the "look back time", which is the difference between the age of the universe at the time that the black holes in #GW190521 merged and the age of the universe at the time that @LIGO and @ego_virgo detected the sources.
@Magrathea_staff @LIGO @ego_virgo After a quick look at @davidwhogg's trusty cosmography guide (arxiv.org/abs/astro-ph/9…), we can calculate a lookback time of 7.15 billion years. This must be where the "7 billion years" comes from.
@Magrathea_staff @LIGO @ego_virgo @davidwhogg It's also worth noting that @LIGO and @ego_virgo use @Planck's 2015 cosmological parameters (arxiv.org/abs/1502.01589) when doing any calculations that rely on cosmology, such as converting the luminosity distance to a redshift.
@Magrathea_staff @LIGO @ego_virgo @davidwhogg @Planck We can plug in all the necessary cosmological parameters in addition to #GW190521's redshift to this handy tool to predict the lookback time, which is called "light travel time" on the tool.
astro.ucla.edu/~wright/CosmoC…
astro.ucla.edu/~wright/CosmoC…
@Magrathea_staff @LIGO @ego_virgo @davidwhogg @Planck And I have to add the usual gripe that the world would be a better place if we could just list the redshift or luminosity distances in our press releases, instead of going through this conversion.
