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THE IB SAYS
THE IB SAYS
Essential idea: The modern field of cosmology uses advanced experimental and observational techniques to collect data with an unprecedented degree of precision and as a result very surprising and detailed conclusions about the structure of the universe have been reached.
Understandings: Rotation curves and the mass of galaxies; Dark matter; Dark energy
Applications and skills: Describing rotation curves as evidence for dark matter; Deriving rotational velocity from Newtonian gravitation; Describing qualitatively the cosmic scale factor in models with and without dark energy
Guidance: Students are expected to be able to refer to rotation curves as evidence for dark matter and must be aware of types of candidates for dark matter
My vodcast
My vodcast
Dark matter
Dark matter
Observational evidence from the Cosmic Microwave Background indicated that the universe is flat. However, when we survey and calculate the density of the material we can observe: stars, galaxies, planets, nebulae, etc. we find that this is significantly below the critical density required for a flat universe. So where is the missing mass (or energy - since according to relativity they are really the same thing)?
We do have evidence that there is matter out there that we can't see. By 'not see' I mean that we have detected no electromagnetic emission or reflection from it, and by matter I mean 'stuff that is affected by gravity', i.e. it distorts spacetime. The first evidence came from the rotation rate of stars in the outer areas of spiral galaxies. If only the visible matter in galaxies was what made a galaxy then stars near the edge would rotate at different speeds to stars near the centre. The spiral arms of galaxies would become wound up. Instead the arms seem to be stable and the stars all rotate around the galactic core with about the same angular velocity. The only explanation for this is if they are embedded in a more massive dark (i.e. invisible) halo (as represented by the blue shading in the picture above).
More recently we have further evidence for, and have been able to map the distribution of, dark matter through the gravitational lensing effects it has when looking at more distant galaxies and quasars.
We do not know what dark matter is, and we have to date not observed any particles of dark matter. Possible candidates for dark matter include neutrinos (often thought to be massless we know believe they sometimes have a very, very small mass and present in large numbers or MACHOs (massive, compact halo objects ... very faint solid objects that exert gravity but are hard to detect). Neither of these options seem to account for the dark matter, leaving us with WIMPs. These are Weakly Interacting Massive Particles, but labelling them brings us no closer to finding them.
Moreover, even when we account for all the dark matter we can map and assume the presence of, the total mass energy, including visible matter is still significantly less than the critical density needed for a flat universe.
Dark Energy
Dark Energy
In the SL section (D.3) on Cosmology we saw that the study of distant Type 1a supernovae revealed that the universe is not only expanding (the Cosmic Scale Factor, R, is increasing) that rate of expansion is increasing (the rate of increase of R is itself increasing). This observation ties in with the missing mass-energy needed for a flat universe. Effectively we believe that this mysterious mass-energy has a property that tends to push the expansion of universe, at least at large scales.
It turns out that this push is also an answer to another problem with our observations regarding the shape of the universe. Since the density of the matter that we observe in the universe (including dark matter) is much less than the critical density, the geometry of the universe should have negative curvature. Yet, all observations we make suggest that the universe is flat - thus there must be an additional mass-energy component. We call this dark energy and it provides that missing component, while also driving the expansion of the universe ever faster through negative pressure.
Thus we are able to outline some properties of dark energy. However, what constitutes dark energy and what other properties it has are still unknown and we do not currently have any good theories of what they might be.
PBS Space Time
PBS Space Time
This four-part video series is the best I've seen on Dark Energy: what it means, why we need it and how we can explain it. Awesome stuff.
Additional videos:
Additional videos:
Shorter than the full PBS Space Time series, but still useful
Resources from textbooks
Resources from textbooks
Oxford Physics: pages 679 - 683
Hamper HL (2014): pages 564 - 566
Relevant questions from Dot Point Physics (E Astrophysics)
Relevant questions from Dot Point Physics (E Astrophysics)
Page 226 - 228
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