Cosmology 101: The Present


Welcome back! Last time, we discussed the first few controversial and eventful moments following the birth of our cosmos. Looking around us today, we know that in the span of just a few billion years, the universe was transformed from that blistering amalgam of tiny elementary particles into a vast and organized expanse just teeming with large-scale structure. How does something like that happen?

Let’s recap. When we left off, the universe was a chaotic soup of simple matter and radiation. A photon couldn’t travel very far without bumping into and being absorbed by a charged particle, exciting it and later being emitted, just to go through the cycle again. After about three minutes, the ambient temperature had cooled to such an extent that these charged particles (protons and electrons) could begin to come together and form stable nuclei.

But, despite the falling temperature, it was still hot enough for these nuclei to start to combine into heavier elements. For the next few minutes, the universe cooked up various isotopes of hydrogen, helium and lithium nuclei in a process commonly known as big bang nucleosynthesis. As time went on and the universe expanded even further, these nuclei slowly captured surrounding electrons until neutral atoms dominated the landscape. Finally, after about 300,000 years, photons could travel freely across the universe without charged particles getting in their way. The cosmic microwave background radiation that astronomers observe today is actually the relic light from that very moment, stretched over time due to the expansion of the universe.

If you look at a picture of the CMB (above), you will see a pattern of differently colored patches that represent anisotropies in the background temperature of the cosmos. These temperature differences originally stemmed from tiny quantum fluctuations that were dramatically blown up in the very early universe. Over the next few hundred million years, the slightly overdense regions in the spacetime fabric attracted more and more matter (both baryonic – the kind that you and I are made of – and dark) under the influence of gravity. Some small regions eventually became so hot and dense that they were able to begin nuclear fusion in their cores; thus, in a delicate dance between external gravity and internal pressure, the first stars were born. Gravity then continued its pull, dragging clumps of stars into galaxies and later, clumps of galaxies into galaxy clusters. Some massive stars collapsed into black holes. Others grew so heavy and bloated that they exploded, spewing chunks of metal-rich debris in every direction. About 4.7 billion years ago, some of this material found its way into orbit around one unassuming main sequence star, creating planets of all sizes, shapes, and compositions – our Solar System!

Billions of years of geology and evolution later, here we are. And there the rest of the universe is. It’s a pretty striking story. But what’s next? And how do we know that all of this theory is even close to correct? Make sure to come back next time to find out!

19 Replies to “Cosmology 101: The Present”

  1. The origin of structure likely has its roots in the breaking of symmetry. If inflation had continued further the universe would have become a thin homogeneous system. However, the process ended with the break down of the inflaton field, or the field which induced the exponential dilation of spacetime after 63 efolds.


      1. the theory of inflation was created by math? do tell…
        i always thought that math was used to support the theory, not create it.

      1. You noticed? Then surely you’d notice the meaningless of your original statement when applied to science. Invoking the old ‘magic’ chestnut can be applied to pretty much anything in science if taken to a deep enough level:

        “Electrons – they magically exist.”
        “The electromagnetic force – it magically exerts forces between charged objects”
        “Quanta – little magical packets of energy that for some reason have a minimum size”
        “Gravity – two massive objects magically attract one another in proportion to some magical quantity called ‘mass’ ”

        Inflation is a well-developed mathematical theory that makes solid, testable observational predictions. It has passed some of these, with others that should be tested soon. Even if we had no idea what started and stopped it (which isn’t really true) it wouldn’t matter a damn – you don’t have to know what causes something to be able to see evidence that it occurred.

      2. hmm.. yes, my choice of the word “magic”… you could sarcastically claim that whatever the most elemental particle is was created by “magic”.

        but since i was referring to what caused inflation to start and stop, i would be more than happy (honestly) to read of these “solid, testable, observational predictions”.

        even if they are not regarding what caused inflation to “start” and “stop”, i would still be happy to read them, just for the info.

    1. No magic in science.

      To quote Wp on inflation: “The hypothetical particle or field thought to be responsible for inflation is called the inflaton.”

      The inflaton field: “Random quantum fluctuations triggered a phase transition whereby [it] released its potential energy as matter and radiation as it settled to its lowest energy state.”

      The initiation of inflation is due to quantum mechanics (in the simplest model) and the ending is due to it achieving its lowest energy state.

      1. well thank you torbjorn for at least giving an authentic response. i am, in an upper-basic-level sense, familiar with inflation theory and even respect much of it.

        i guess my little outburst was mainly due to frustration with what i view as the modern tendency to accept non-conclusively proven theories as LAW. you could consider me a very conservative thinker in this area of research.

        what i see in the era of the internet (and sims) is a rapid acceptance of new theory (this theory is actually from the 80’s) on an unprecedented scale; almost as if with a religious zealotry by the people who themselves deride religious zealotry.

        my purpose is not get into flame wars with other posters. that is simply a by-product of people not liking my view. my main goal ALWAYS is to try to influence undecided readers to never be afraid of questioning modern theory. we’d still be in the dark ages without critical thought. there is little doubt that much of what we believe in the present time will be heavily revised in the future, so don’t be afraid !

  2. What kind of light existed back then?

    I mean infrared didn’t exist? So, we would detect it as radio today.

    1. Same light as today.
      The CMB is a thermal radiation which was emitted by matter at 3000K: its frequency peak was in visible light, which redshift reduced to microwaves.
      The infrared (redshifted to radio) component would be very weak by comparison.
      If I got your question right ;D

      1. Thanks, I was aiming at the thing that you always hear CMB, but then I wonder why there is no CRB…

  3. Folks: Light is light. Optical radiation is between 300 and 600 nm, whether that is now, in the first seconds of the big bang or 20 billion years from now. Electromagnetic radiation of all bands or wavelengths has always existed. Optical radiation produced at the end of the plasma or radiation dominated period is now redshifted by a z ~ 1000 into the microwave band.

    The inflationary period is due to a large vacuum energy with a negative pressure. This is associated with a dilaton type of field. Inflationary cosmology starts us off at that point. The breaking of this symmetry takes us to the collapse of the vacuum energy density to a very small value, the conversion of this energy to heat, and where that heat is what we identify as the big bang. The reasons for this set up initially have to do with issues of quantum vacuum structure and string compactification and other things. The chain of explanation so far only goes a certain time back. Yet that is how science works. Theories explain something, not necessarily everything.


  4. The fact of the matter is that we do not really understand how the universe came about. We cannot revisit that time, so obviously any scientific descriptions we form have to be based on how the rest of the universe operates and our detailed understanding of the constitution of matter and the laws governing its interaction. The hardest features of the universe for us to intuitively understand are the relativistic theories which seem to govern how matter, energy and space-time operate under extreme conditions of temperature, density and energy.

    The issues of where the Big Bang occurred and how it happened seems to be locked up in these unfamiliar and very subtle features of the physical world. It is impossible for a non-mathematician, using the best crafted essays, to completely understand what it has taken decades of mathematical/logical abstraction and intense experimental analysis to create.

    The best, non-mathematical description that any cosmologist can create for describing the Big Bang is that it occurred in every cubic centimeter of space in the universe with no unique starting point. In fact, it was an event which our mathematics indicate, actually brought space and time into existence. It did not occur IN space at a particular location, because it created space ( and time itself) as it went along. There may have existed some state ‘prior’ to the Big Bang, but it is a state not described by its location in time or space. This state preceded the existence of our time and space.
    So… I’m going to bed.

  5. The inflationary concept (in the Inflationary hot Big Bang Model) , supporting a vacuum-dominated universe (arising out of quantum fluctuations) during phase transition in the early history of the universe was evolved by some cosmologists (Guth 1981; Linde 1982) to circumvent problems of ‘flatness’, ‘horizon’ and the ‘primordial magnetic monopole’ associated with the Big Bang model. The hypothetical inflation field giving rise to inflation still remains very speculative.The beginning and end of the inflationary epoch remains highly conjectural. There is no general consensus yet among the cosmologists regarding the timing of the beginning and end of the inflationary epoch. In Linde’s ‘chaotic inflation’, inflation starts at the Planck time ,10 -43 sec when the temperature was 10 32 K, whereas in other models, inflation starts when the temperature falls to the point (10 -35 sec after Big Bang when the temperature was ~10 28 K) at which the symmetry of the Grand Unified Theory (GUT) is spontaneously broken.

    Element of arbitrariness is also quite prominent in regard to interpretation of the cosmological costant in Einstein’s equations of general relativity. Some cosmologists have related the cosmological cosnstant to the dark energy following observations in 1998 of very distant galaxies that were suggestive of accelerating expannsion of the universe. Ironically, the true nature of the 73% dark energy and 23% dark matter
    ( as per the latest WMAP intertretations) pervading the universe hitherto remains elusive so far.

  6. If the force of the expansion was greater than the force of gravity keeping the universe from collapsing on itself like a black hole, wouldn’t it also overpower the other actions like trading electrons and such, how could they “come together” if the force of expansion is greater than the force of the universe collapsing into a black hole?

    1. “Some small regions eventually became so hot and dense that they were able to begin nuclear fusion in their cores; thus, in a delicate dance between external gravity and internal pressure, the first stars were born”

      Why did this not happen before when matter was already infinity (as a figure of speech) more dense beforehand? It seems like in these discussions gravity is left out, then it is discussed and put into the equation when it starts making sense to us (gas clouds forming into stars, planets forming etc).

  7. There are a couple of errors in the article: It says “After about three minutes, the ambient temperature had cooled to such an extent that these charged particles (protons and electrons) could begin to come together and form stable nuclei.” Obviously protons and electrons don’t form nuclei! It should probably say “some neutrons were captured by protons to produce the nuclei of helium, deuterium and traces of lithium.”

    The second section gets closer but the nuclei captured electrons to form neutral atoms around 378,000 years after the bang which released the CMBR. Prior to that, the temperature was too high so the plasma was all ionised. The quote gives the false impression that capture was a slow and perhaps cumulative process.

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