4 clarifications about the history of the universe

James Peebles

Certain statements by James Peebles, recent Nobel Prize in physics, have aroused controversy, although what he said is not something new, as theoretical physicists have long been saying precisely the same thing.

The Big Bang theory was proposed in 1931 by Georges Lemaître, by extending to the past the Hubble-Lemaître law. In 1948, Ralph Alpher and Robert Herman predicted that, if the Big Bang theory is correct, there must be a cosmic background radiation with a temperature close to 5 Kelvin. In 1965 Arno Penzias and Robert Wilson discovered the existence of such cosmic radiation, whose temperature proved to be 2.72548 Kelvin. The temperature is exactly the same in all directions, except for two effects that cause small differences, but never affecting more than the third decimal place.

The cosmic background radiation represents the largest distance in the universe that we can detect directly. Anything further away is hidden, for the universe was then a plasma and was opaque, so we cannot see it. The situation resembles what happens with the sun: we can only see its surface, for its interior is opaque and out of our reach; we just can make deductions about it.

The name of the Big Bang theory was mockingly invented in 1950 by its opponent Fred Hoyle. The name was accepted by its supporters, although it was not very appropriate, as the word Bang suggests that what then happened was something like an explosion. This has given rise to many misunderstandings. A more appropriate name for the Big Bang which does not arouse misconceptions, is the initial singularity.

Let’s provide a few clarifications:

1. The expansion of the universe, which began in what is incorrectly called the Big Bang, does not resemble the result of an explosion. There are no objects (galaxies) in the universe thrown into space at a certain speed by an initial impulse, departing from one another. The space itself expands. If all the galaxies were in perfect rest, they would still separate from each other, because the space between them is lengthening. And it expands equally in all directions. There is, therefore, no center of the universe from which all galaxies depart.


2. A good way to imagine what happens with space is to think about the surface of an inflatable globe, a balloon with a map of the Earth pictured on its surface. While the balloon swells, each point on the map is moving away from the others at a speed that depends on the distance that separates them, not because the geographical points are actively moving away from each other, but because space itself (the surface of the globe) increases in size. This is a two-dimensional model of what happens in the universe in three dimensions. Note that on the surface of that globe there is no center from which all other points depart.


3. Actually, galaxies are not at perfect rest. Their mutual gravitational attraction makes them move through space. Thus, our galaxy (the Milky Way) and the Andromeda galaxy move towards each other at the speed of about 100 km/sec, so it is possible that both will collide and merge in a few billion years. And the local group of galaxies, to which these two giant galaxies belong, is attracted by an even larger group of galaxies (the Virgo cluster), which in turn is part of the Laniakea supercluster.


In addition to all these movements, typical of our galaxy, we must add a few more, which affect the Earth:
• The Earth turns around its axis at a speed of 0.465 km/sec on the equator.
• The Earth revolves around the sun at a speed of about 30 km/sec.
• The sun revolves around the center of the Milky Way galaxy, dragging the Earth at a speed of about 220 km/sec.
• The Milky Way galaxy and the entire local group move, attracted by other galaxies, at a speed of about 600 km/sec with respect to the cosmic background radiation.
4. As a result of all these movements, which oppose one another and partially compensate, the Earth moves with respect to the cosmic background radiation at a speed of about 360 km/sec. This movement has the effect that a part of that radiation (the side towards which we are heading) appears slightly warmer, while the opposite side seems a bit cooler. The attached figure, taken from this article, shows this effect, which affects the third decimal figure of the temperature of the cosmic background radiation. The figure amplifies the effect, showing in yellow the area where the temperature is higher (2.728K) and in blue the area where it is lower (2.722K).

Microwave Cosmic Background Radiation

Once this effect is eliminated, there are still small differences in the temperature of the background radiation, affecting the fifth significant figure. These differences represent real variations of the background radiation itself, which correspond to areas where the plasma visible in the radiation was slightly denser or slightly less dense than the surrounding areas. The existence of galaxies is attributed to these minimal initial differences.

The same post in Spanish
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Manuel Alfonseca