Answer to Question #93426 in Astronomy | Astrophysics for Edward Barrett

Question

I understand Hubble noticed (or perhaps published) the fact that the universe is expanding in 1929.

I also understand that the brightness of 'standard candles' (such as Cepheid variable stars) can be used to calculate their distance from us.

My questions are - how much further from us are things now than they were when the distance was measured in 1929?

Do calculations using standard candles completely agree with Hubble's findings?

Thanks.

Answer 1

I'll answer your last question first. Calculations with standard candles give various values for Hubble constant. Below are some of them.

1994: 3 cepheids from Virgo constellation gave the value of 87±7 km/s/Mpc.

13 cepheids from M100 Galaxy gave the value of 80±17 km/s/Mpc.

A supercluster from Coma Berenices gives value of 77±16 km/s/Mpc.

Thus, it's up to you to decide whether these results completely agree with conversational value of:

66.93±0.62 km/s/Mpc in 2016,

67.4 ± 0.5 km/s/Mpc in 2018.

The reasons why these values (even within the same year) differ for more than 3 sigma aren't yet fully explained.

Now the answer to the first question. Since Hubble constant depends on time, and the distance to the objects depends on Hubble constant, it's hard to say how much farther from us these objects became within past years. The answer depends on what objects we are taking about, because Hubble's law is not applicable to all galaxies because they may have significant peculiar speeds. When Hubble discovered this phenomenon, he estimated the value to be 500 km/s/Mpc (later Jan Oort thought that something was wrong with that result and published a value of 290 km/s/Mpc). Moreover, Hubble calculated this when some phenomena were not discovered, and he made calculations for the local supercluster with peculiar speeds which he didn't take into consideration.

If we take these values and velocity for a galaxy with red shift 0.5 (or 115000 km/s), we'll get that for 90 years from 1929 to 2019 the object is further for

$$∆d=115000\Big(\frac{1}{67.4}-\frac{1}{500}\Big)=1476\text{ Mpc}.$$

But keep in mind two things: 1) this result is overestimated, 2) now galaxies are further than they were when Hubble was alive.

Hubble's law describes how further galaxies become with time because of expansion of the universe (space), while all of them have their own (peculiar) velocities of hundreds or thousands km/s which describe how fast galaxies travel with regard to the local space (Milky way or the sun for instance). To obtain less incorrect value, one should add or subtract these velocities from the velocity we use in Hubble's law.