PHY111 Our Evolving Universe:
Lecture Summaries

The first map of the microwave sky produced by ESA's Planck spacecraft. The blue areas of the map are emission from gas and dust in our Galaxy – the line through the middle of the map is the Galactic plane. The reddish areas towards the north and south poles of the map show the cosmic microwave background – its reddish "colour" (obviously the colours are artificial, as this map is not in visible light) indicates that it is at a lower temperature than the Galactic emission. The subtle variations in the microwave background are currently our most productive source of data on cosmology.

Summary of Lecture 11 – The First 400000 Years

1. In the first 10^-35 seconds of the Big Bang model, the Universe is born as a sea of very high-energy elementary particles:
   • current theories of particle physics imply that the physical laws were different in the very early universe:
     • our present four forces (strong, electromagnetic, weak, gravity) were combined into one superforce;
     • particles and antiparticles existed in equal numbers, constantly annihilating into energy and being recreated from energy (E = mc²);
     • there were no protons or neutrons, but only the elementary particles that we call quarks (together with electrons, neutrinos, photons, etc.);
   • the transition from this very high energy physics to 'our' physics powered a brief period of very rapid expansion (inflation);
   • inflation explains three issues that are hard to explain in the original big bang model:
     • the horizon problem – the fact that widely separated regions on the sky have the same microwave background temperature, even though it looks as though they could never have exchanged photons;
     • the flatness problem – the fact that the geometry of the universe seems to be very close to flat, though there is no theoretical reason for this;
     • the small variations in the density of the very early universe, which are the "seeds" from which the galaxies we see today will eventually grow;
   • inflation solves these problems because:
     • our visible universe originated in an extremely tiny region of the pre-inflation universe, which had time to stabilise its temperature before inflation started;
     • the inflationary expansion is so rapid that it flattens out pre-existing curvature;
     • tiny random quantum fluctuations expand in size during inflation to become marginally denser regions of the early universe;
2. After about 1/10000 second protons and neutrons form:
   • for reasons that are not yet understood, there is a slight asymmetry between matter and antimatter, so the resulting particle-antiparticle annihilations leave some protons and neutrons (but no antiprotons or antineutrons) left over;
   • because the neutron is slightly heavier, there are about five times as many protons as neutrons;
   • because free neutrons are unstable, this ratio will increase with time as the neutrons convert to protons (emitting an electron and a neutrino).
3. After about 1 second the protons and neutrons start to combine to form deuterium (²H):
   • initially this just gets broken up again, but after about 100 seconds it starts to build up as the universe cools;
   • the deuterium then combines with a proton or neutron to form ³H (tritium) or ³He;
   • these in turn collide to make normal helium, ⁴He;
   • eventually almost all the remaining neutrons are incorporated into ⁴He:
   • for every 2 neutrons there are about 14 protons (allowing for neutron decays);
   • therefore for every helium nucleus there are 12 hydrogen nuclei: the universe is about 25% helium by weight (with traces of deuterium and helium-3 left over, and a bit of ⁷Li formed by colliding ⁴He with ³He);
   • the lack of stable nuclei with atomic mass of 5 or 8 stops the process from continuing to heavier elements.
4. The Universe is now a mixture of hydrogen and helium nuclei, electrons, photons, neutrinos and dark matter:
   • the photons interact vigorously with the charged particles, producing a thermal (blackbody) distribution (the early universe is hot and dense);
   • but after about 380,000 years, when the temperature is about 3000 K, the electrons combine with nuclei to produce neutral atoms:
     • these do not interact so readily with photons;
     • the Universe is now transparent and pervaded by a sea of blackbody radiation;
     • this radiation, redshifted to 2.7 K by the expansion of the Universe, can now be detected as the cosmic microwave background radiation;
     • the tiny density variations produced in inflation can be seen as temperature variations in the CMB:
       • higher density implies slightly greater gravitational attraction, so these regions collect more mass and become denser still;
       • eventually they give rise to galaxies, stars, us...

Web links

• The PowerPoint file for this lecture.
• Nick Strobel has a chapter on cosmology; Gene Smith has a couple of pages, including a convenient timeline. There is also Ned Wright's Cosmology Tutorial, which is excellent, although the tutorial itself is aimed a bit above our level (the FAQ page is OK).
• A short self test for this lecture.

Go back to main page
Go on to Summary for Lecture 12