The Big Bang model

CMB spectrum (COBE)

Cosmic Microwave Background spectrum (credit: NASA)

The Big Bang theory describing the origin and expansion of the universe from a very tiny and energetic initial state was developed initially in the 1920s as a solution for Einstein’s equations of general relativity. It assumed, correctly, a uniform (homogeneous) density of matter and energy. While the universe around us today appears highly non-uniform, with visible matter apparently concentrated in groups of galaxies, and in individual galaxies, gaseous nebulae, and star clusters, stars, and planets, all the evidence indicates that matter was very uniformly distributed throughout the first one million years of existence. At that time there were no stars or galaxies, rather the universe consisted of hot dense, but expanding, gas and photons (light). Even today, on the largest scales of 500 million light years and beyond, the universe appears to be quite uniform on average.

The first great support for the Big Bang came from the detection of what we call the Hubble expansion, named for Edwin Hubble, who in 1929 first demonstrated that galaxy recession predominates and depends on distance from us. Galaxies on average are all moving away from each other, unless they are gravitationally bound to their neighbors. The rate of expansion is simply proportional to the distance to the galaxy; this is known as Hubble’s law. Every galaxy moves away from every other galaxy regardless of its position in the universe; this implies a global and uniform expansion.

How do we determine this relationship? The light from these distant galaxies is shifted to be redder than normal in proportion to the velocity away from our galaxy. The redshift is a measure of the velocity of recession and the velocity is found to be proportional to the distance from our Milky Way to the galaxy in question. To be clear, the galaxy velocity and distance follow a linear relation. If we were located in another galaxy, we would observe the same effect. Most of the galaxies would be receding from us as well, at rates proportional to their distance. This is just what one expects for a universe which is isotropic – the same in each direction – and which is expanding uniformly. Each dimension of three-dimensional space is getting larger with time. The gravitationally bound objects, such as the galaxies themselves, are not expanding, but the space between the galaxies is stretching and has been since the Big Bang initial event.

Since the rate of the expansion is proportional to distance, one can take the proportionality constant, known as Hubble’s constant, and by inverting that determine an approximate age of the universe. It amounts to ‘running the movie backward.’ The age works out to 14 billion years, which is very close to the current best estimate of the age of 13.8 billion years, about 3 times the age of the Sun and the Earth.

Another great success of the Big Bang model was in its prediction of the helium abundance. The same hydrogen fusion process that powers the Sun took place in the early universe during the first 20 minutes, when the temperature was millions of degrees. In the Sun hydrogen is fused to created helium. For the early universe, this is known as primordial or Big Bang nucleosynthesis. There was only time enough and the right conditions to create helium, the second lightest element in the periodic table, and also the heavy form of hydrogen known as deuterium, plus just a bit of the third element lithium. None of the heavier elements such as carbon, nitrogen, oxygen, silicon or iron were created – this would happen later inside stellar furnaces. The final result of this cosmological nucleosynthesis turned 25% of the initial available mass of hydrogen into helium, and into trace amounts of deuterium, lithium and beryllium. The primordial abundance observed in the oldest stars for helium and deuterium matches the predictions of the Big Bang nucleosynthesis model.

The Big Bang moved from being possible theory to well-established factual model describing the universe when the first detection of the cosmic microwave background was published in 1965 by Arno Penzias and Robert Wilson, who received the Nobel Physics prize for their discovery. The cosmic microwave background is blackbody thermal radiation at millimeter wavelengths in the radio portion of the electromagnetic spectrum., and as we observe it at present, it has a temperature of a little under 3 degrees above absolute zero (see image above which has the characteristic thermal blackbody shape). It fills space in every direction in which one observes, and is remarkably uniform in intensity. The cosmic microwave background dates from a time when the universe was about 380,000 years old, and the radiation was originally emitted at a temperature of around 3000 degrees on the Kelvin scale. It also has redshifted, by over 1000 times. Thus we detect today as radio waves photons that were originally emitted in the optical and infrared portions of the electromagnetic spectrum when the universe was only 380,000 years old. Unlike the hydrogen and helium atoms which are found in stars and on planets, these photons have stretched out in proportion to the expansion of the universe.


5 responses to “The Big Bang model

  • Alex L

    Cool blog! I found it through I’ve been looking for a blog that distills the rarefied world of cutting-edge astrophysics into terms a lay person (like me) can understand. I’ll be subscribing to your articles.

    • darkmatterdarkenergy

      Alex, thanks. If there is a question or a particular topic of interest in cosmology, please post and I will try to respond.

      • Harrison


        Great blog, however I am still lost concerning a subject. The universe is about 13.8 billion years old from us, but if we were in a near by or distant galaxy, would we still gather the same age for the universe? Or are all galaxies at the same distance from the edge of the universe/where the big bang occured?
        Also (I know it’s a lot already, sorry!), Do we expand out with the edge of the universe, and if not, how can we be sure that the age is correct if we become more and more distant from the edge? Assuming that we use the light of an edge of some part of the universe to calculate its age.

        As you can see, I am really confused about how we determine the age, and how we know where to point our cameras.

        Thank you for your patience with these questions, I appreciate it!

      • darkmatterdarkenergy

        All good questions. Don’t think of an explosion from a point, think more of space unfolding and flattening and stretching an incredible amount in each of the 3 familiar spatial dimensions. Space itself can expand faster than the speed of light, and does in fact do this, general relativity allows it. Objects within the space cannot move faster than the speed of light relative to their local surroundings.

        In a near or distant galaxy still same age of universe?

        All nearby galaxies would observe the same 13.8 billion year age for the U. A galaxy one billion light years away would also measure the same age “now” from its own position, but the image of the galaxy that we see today is a billion years younger than “now” due to the light travel time.

        Are all galaxies at same distance from where big bang occured?

        The Big Bang didn’t occur in one place, it wasn’t an explosion in space, it was everywhere in the universe. A bubble of space became the universe and during the inflationary epoch was driven to expand from incredibly submicroscopic up to macroscopic scale. And it continues to expand more gradually than during the inflationary era, yet accelerate again due to dark energy. In their local frames of reference all galaxies are 13.8 billion years old, or less, depending on when they formed.

        The usual analogy is if an ant lives on the surface of a balloon which is expanding; it is confined to the surface and doesn’t see the interior. It does see the other ants are moving away from it as the balloon grows in size. Just as we travel pretty much on the surface of the Earth and don’t go into the interior. Imagine if the earth were doubling in size every hundred years!

        Or think of just one dimension, a tiny piece of string, then stretch it rapidly to be incredibly long, doubling again and again in length hundreds of times. Some ants living on the string will be neighbors, others will be far from one another, and all will move relatively away from each other as the string stretches farther. There could be ends to the string, but this gets into string and M-theory. Who knows where the middle is? We have no way of telling since most of the string would be beyond our window of visibility (light horizon).

        Do we expand out with edge?

        We’re not at the edge, we’re not in the middle, we don’t know where we are. But in the inflationary Big Bang the universe is incredibly larger than the portion that we see, perhaps 25 orders of magnitude or more. The expansion causes our galaxy to continually move farther away from our neighbor galaxies, unless we are gravitationally bound to them.

        How can be sure age is correct if we’re more distant?

        We measure the age by a few methods. One is the expansion rate of galaxies, which allows us to extrapolate back to the time when the universe would have been infinitely small. A second method is through measuring the microwave background radiation. We know from the physics that the radiation was originally at 3000 Kelvins, now it’s less than 3 degrees since the radiation is stretched out in proportion to the scale factor of the universe. Thus the universe has expanded by 1100 times in each dimension, or over a billion times in volume. A third method is the ages of the oldest stars, which are consistent with the 13.8 billion year age of the universe.

        On very long time scales your question is pertinent, since In the very distant future it will get harder to measure the universe’s age because “we” (our descendants hundreds of billions of years from now) won’t be able to see other galaxies, they will have moved outside “our” light horizon! And the microwave background radiation will be fainter and fainter and harder to detect. Today is truly a golden age for cosmology.

  • Harrison

    Thank you very much for taking the time to explain and for elaborating! It is all much clearer now, and I believe I have a much greater understanding of it all.

    Most of these questions were due to a need for knowledge and understanding gained from seeing a show called “Through the Wormhole”; So once again, with great appreciation, thank you!

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