التوسع الكوني وقانون هابل

a120

About the Hubble's law they use a gas system which expands according to this law : a bubble emitted by an aqualung. When it reaches surface, the pressure becomes smaller and this bubble expands.Its size is proportional to the inverse of the pressure. If some observer was sitting on a molecule and observed the other, they should own, with respect to this observer, an escape velocity propotional to distance. Subaquatic activities has something to do with cosmology.

Universe is assimilated to an expanding gas. But random "proper move", corresponding to local turbulence, introduces a "noise" when one wants to measure the radial velocity, corresponding to a given distance d. For example, when Hubble measured the radial velocities of neighbouring galaxies, like Andromeda, which moves towards us, he concluded that the universe was shrinking. But it was just a local artifact, due to random velocities.

In the air you breath molecules have a random (thermal velocity) close to 400 m/s ( notice it is close to the sonic velocity).

Similarly the random velocities of galaxies, in their clusters, lies between 500 to 1000 km/s. So that if one wants to evaluate correctly the escape velocity, he has to choose a sample of galaxies far enough to have a radial escape velocity higher than this thermal velocity.

The mesurement of radial velocity is very easy and accurate. It is based on the red shift. But the difficulty is to evaluate the distances. How far are theses galaxies from us ?

Historically, the galaxies were first called "nebulae". Some people thought they were located inside our milky way. But Hubble himself answered this question, finding a cepheid in Andromeda, which was pushed at cosmological distant : more than two millions light-years far from us.

At large distances it was very difficult to isolate a cepheid in these gloomy spots.Then , the distance evaluation was a question of opinion. During years the evaluation of the Hubble's constant was controversial. Many tended to values which fit the 15-20 billions years of the oldest stars of our galaxies (from globular clusters). In US, the French astronomer de Vaucouleurs argued and tended to have different values.

All the eyes were fixed on the space telescope Hubble. In 1993 this one, with its sharp single eye, discovered cepheids in very distant galaxies, located at ten million light years. At such distance the random velocities were basically masked by the general radial velocity.

• Great, said the astronomers. We will have an accurate measure of the Hubble's constant Ho !

But, rapidly, the adventure transformed into a tragedy. From 1993 to 1994, Hubble's measurement of these galaxies, 48 and 55 light years distant, were 40 % found closer by (with respect to oldest distance evaluations) ! So that the age of the Universe became 40 % shorter. In 1994 Nature talked of 8-9 billion years......

He was younger than its own stars. Immediatly the astrophysicist and cosmologists tried to save the old good Standard Model and turned immediatly towards the cosmological constants, like firemen, facing a an intellectual cosmic fire, searching nervously for an extinguisher.

This constant was rapidly pulled out of its naphtalene and reactivated. On the figure (108) you can see the impact of this constant on the expansion.

(108)

Some astrophysicist said :

• These new measurements, performed by Hubble space telescope, is nothing but a method to scale the cosomological constant. Now, we know its value !....

Few years later, what about this problem.

Two new facts : The satellite Hipparcos had performed accurate measurements on distants stars, based on parallax. Then astronomers changed the cepheid calibration.

• The objects are farther than we thought before. We have recalibrated than.

Andromeda was pushed away, from 2.5 billions years to 3.5 billions years.

• Wouffff.....

The evaluation of the age of the oldest stars of our galaxy was revised too. Everybody did his best.

By the way, the model "universe interacting with a ghost universe" (see Geometrical physics A) gives another decoding of Ho's measurements, giving an oldest universe.