Thursday, January 27, 2005

Dark energy

Below is a figure I stole from Sean Carroll at the University of Chicago. For those who haven't yet heard, dark energy is a strange substance with negative pressure, whose existence has been recently inferred from cosmological observations. It seems to constitute about 70% of all the energy in the universe, and is causing accelerating expansion. (Don't worry though - nothing very noticeable will happen for the next few billion years.)

Theorists have no idea what this stuff is. Let me repeat that - theorists have no idea what this stuff is! The discovery, via supernova distance-redshift observations, was totally unexpected. That makes studying it tremendously exciting. However, we may not find out what it is for a long time. For example, dark energy might be a quantum field with very weak interactions, coupled to us only via gravity. In that case we might not learn much beyond how it affects the large scale evolution of the universe.

If the discovery of dark energy holds up, it will be one of the most surprising and important discoveries of the last century. The public hasn't really caught on to this yet, but who can blame them with all the background noise about speculative (meaning, very likely not real) things like superstrings, extra dimensions, supersymmetric partners, etc.


Anonymous said...


How true!

BTW, what about dark matter (25%; 5% the visible portion)? Can that be explained by massive neutrinos (a minor modification of the original standard model) or does that also require physics beyond standard model (like dark energy/non-zero cosmological constant)?

A less appreciated fact by people outside the field is the intimate relation between particle physics and cosmology. I think talking of "building blocks" hides the subtle and revolutionary idea of the quantum field. Thus, there is an electron (or photon) field, whose excitations is the electron (or photon). Thus, one electron is *identical* to another electron anywhere else in the universe, which leads to peculiarly quantum phenomena based on Fermi (or Bose for photon) statistics. In contrast, no two macroscopic objects are ever identical.


Anonymous said...

"For example, dark energy might be a quantum field with very weak interactions, coupled to us only via gravity."

What does this sentence mean?


Nice post.


Steve Hsu said...


As yet another sad commentary on how slowly fundamental physics progresses (relative to human lifetimes), we still don't know what the dark matter is. It could be neutrinos, it could be superpartners, it could even be tiny relic black holes. I predict we will know what DM is long before we know what DE is, though.


"a quantum field with very weak interactions, coupled to us only via gravity"

All matter and energy is a manifestation of some quantum field (according to current physics). We can only detect something if it interacts with us (i.e., can exchange energy with ordinary atoms or photons). It is possible that the only manifestation of dark energy that affects us is its gravitational pull (it only interacts directly with the gravitational field, but not with atoms or photons). If so, we will have a very hard time probing it to discover its properties.

So, the discovery of dark energy via its large scale (cosmological) consequences might be yet another tantalizing clue about the universe that we are unable to follow up on in a concrete way (leading to more unsupported speculation by theorists)...

Anonymous said...

Yes; I understand at once. You and MFA are strikingly clear.


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