The Steady-State Universe
Today we all know the Big Bang Theory. But it was not always accepted as the sole theory to explain what goes on in the Universe: up to just 60 years ago, there was another. Here is its story.
We all have heard of the Big Bang Theory, at least once. We all have deep into our imagination the depiction of the birth of the cosmos like the explosion (wrong) of a little atom (wrong) at the instant zero of time (wrong).
Well, this is a little far from the actual theory, hence all the wrong instances in the last sentence.
Nevertheless, we seem to grasp the basic idea, the fact that the Universe was once smaller (and hotter) and denser and that if we go enough back in time, we will reach a point (in time, not a physical one) where laws of physics break down: this is known as the Planck scale, and has a size of 10-to-the-minus-35 m1; a temperature of 10-to-the-322 K3 , and a mass of 10-to-the-minus-8 kg4 .
Even at this scale, things are not quite clear. Below this scale, we don’t know what happens.
The Big Bang Theory says that the Universe expanded, things got cooler, space got bigger and less dense, and in about fifteen minutes there was Hydrogen and Helium (and some trace of Lithium, alongside isotopes of Hydrogen and Helium).
Okay, this is the BB theory. What about the Steady State Theory? What is this theory, what does it describe, and most importantly: why today is not accepted as the standard theory?
Why so steady?
The idea behind the Steady-State model is the following: consider the Universe as a whole, at the scale of hundreds of Megaparsecs5 ; the Universe at this scale is rather homogeneous and isotropic, meaning that is made up more or less of the same things (homogeneous) and that looks the same in every direction one observes it (isotropic).
So, with this consideration (named the Cosmological Principle) the Universe has these two properties in space. But is it true also in time?
This is a question answered by the Perfect Cosmological Principle, that states the homogeneity and isotropy of the Universe both in space and time.
The Steady-State Theory of the Universe follows from this principle.
This theory was developed in 1948 by H. Bondi and T. Gold, and Fred Hoyle in two separate articles.
Embracing the Perfect Cosmological Principle, Bondi and Gold state that the universe would expand, because an infinitely old static universe would reach some point of thermal equilibrium and everything would be at the same temperature (which is not the case). Technically there would be the possibility of a contracting universe, but they discard it due to the excess of radiation over matter that this would imply.
Then they move on to the observed evidence of distant galaxies, whose velocities are interpreted as the motion of expansions (proportional to distance). In a frame of valid hydrodynamic continuity then, density matter should decrease. Hence it is clear that to have a static Universe matter must be continuously created. The rate of creation can be estimated to be at most one proton per liter per billion years. (it’s not so much matter anyway).
In this type of Universe, the velocity between distant objects (cosmological distances, to be precise) has to increase as the distance increases.
Furthermore, the ratio between uncondensed and condensed matter must remain constant, so that new galaxies form as older ones move away from each other. In this depiction, one observer could not be able to distinguish a “universal cosmic time”, or derive a unique description of this quantity6.
It’s natural to grasp the logical evidence that the age distribution of galaxies in any volume will be independent of the time of observations (the Universe is always “the same”!) and it will hence be the same for both distant and near galaxies. This was the master way to construct the interpretation of observations of distant galaxies.
Another point they discussed is the thermodynamics of the Universe and the evidence that our Universe is in thermal disequilibrium. To obtain this, one would need an expanding Universe. The recession of nebulae (as stated in the paper in 1948 - we now know they are galaxies) indicates then the correct explanation for thermal disequilibrium - i.e. expansion.
Observational Tests
The idea behind the observational tests necessary to test this theory is that relying on the electromagnetic spectrum is not possible, due to the limitations that this theory has in regard to the field equations. Therefore the observational tests rely largely on kinematical arguments, assuming total homogeneity and isotropy.
Nevertheless, the tests resulted in similar problems as the ones from different (and “standard”) cosmologies: data were incomplete and loose enough to contemplate the possibility that either of the theories was correct.
With the advent of radio telescopes (that were largely improved in the 1950s and consolidated in the 1960s), observations showed that the number of radio sources (objects that emit primarily in the radio band of the electromagnetic spectrum) was not constant with the distance from the site of the observations, meaning that there were more distant radio sources far away from the Milky Way than the theory predicted. This meant that there were more radio sources in the past: the Universe was changing in time!
The end was not near: another class of astrophysical (cosmological, to be fair) objects did discredit the steady-state theory: it is known as a quasar, a type of active galactic nuclei (AGNs), that are galactic cores where gas and dust that fall into the black hole at the center of the galaxy emit electromagnetic radiation. They are extremely luminous but are found only in the early universe. We don’t see a quasar nearer than 600 million light-years from Earth.
The final observation that falsified the steady-state theory was the discovery of Cosmic Microwave Background Radiation in 1965. This is the electromagnetic radiation emitted roughly 300000 years after the start of the metric that describes the Universe in the Standard Model (the start of the metric is also known as the Big Bang, but it was not Big and was not a Bang) and today peaked at the microwave, due to the expansion of the Universe. This means one more time that the Universe does change over time, and that the only Cosmological Principle that seems to fit is the plain one, not the Perfect one.
It was a good run
Scientific theories must be falsifiable. They must provide explanations for some phenomena, and possibly make predictions: only this way knowledge can go forward.
The Steady-State Theory had the audacity to propose an infinite, never-changing universe: these qualities were disproven.
It certainly has some fascination to think about the Universe as this infinite sea of perfect homogeneity, where the absence of a center is sostituted to the awareness that this portion of the Universe is replicated in its general properties someplace else. Unfortunately for the theory and its supporters, it does not seem to be the right one (as we could ever find it, a “right” one).
Some links for further reading:
H. Bondi , T. Gold, The Steady-State Theory of the Expanding Universe, Monthly Notices of the Royal Astronomical Society, Volume 108, Issue 3, June 1948, Pages 252–270, https://doi.org/10.1093/mnras/108.3.252;
F. Hoyle, A New Model for the Expanding Universe, Monthly Notices of the Royal Astronomical Society, Volume 108, Issue 5, October 1948, Pages 372–382, https://doi.org/10.1093/mnras/108.5.372;
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for comparison, an atomic nucleus has a typical size of
to get a sense of this number, note that the temperature of the Sun’s core is
Kelvin, one way of defining temperature (if you ask a physicist, THE way to measure temperature). To convert from K to °C all you need is subtract -273.15 °C. You can understand that at the high temperature considered here, the distinction is somewhat unimportant.
I won’t pester you anymore with scientific notation; for comparison the mass of a human cell is
Cosmic time: is the time one observer could measure if bound to the expansion of the Universe, hence with zero peculiar velocity. You can imagine it like the time that is associated with the expansion of the Universe.
Did you imagine it? Very good.