Web edition: June 17, 2011
Print edition: July 2, 2011; Vol.180 #1 (p. 30)
Your cosmic questions
Regarding the “The vital statistics” in “Cosmic questions, answers pending” (SN: 4/23/11, p. 20), I was puzzled by two values: 13.75 billion years (time since the Big Bang) and 90 billion light-years (diameter of the universe). If light has been streaming away for 13.75 billion years, then shouldn’t the diameter of the universe be 27.5 billion light-years? Or is the outer two-thirds of the universe populated with something moving faster than the speed of light?
Mark Brown, Littleton, Colo.
As the age of the universe is 13.75 billion years, it has taken light emitted at the Big Bang that long to reach observers on Earth today. But during that time, the universe has been expanding, and space’s expansion is not restricted by the speed of light. An object that emitted light that arrived at Earth 13.75 billion years later would now be roughly 45 billion light-years away, making the current diameter of the observable universe about 90 billion light-years. — Tom Siegfried
Your April 23 issue dealing with current issues in cosmology was one of your best. Fascinating! As a layman I have always been unsettled by the seemingly strange contrivances such as dark matter and dark energy used to explain the observations made by astronomers. As an alternative, could there be a problem in the interpretation of the observations of the radiation from stars and galaxies on which such theories are based?
George Sutherland, Sammamish, Wash.
Astronomers have long considered such possibilities, but observations repeatedly rule out most alternative interpretations. There always remains some small possibility that the observations have been misinterpreted. — Tom Siegfried
If dark matter makes up 85 percent of matter in the universe, and it is the “fill” between galaxies, wouldn’t there be friction when traveling through this much “matter”? Or could we be confusing matter with energy at the subatomic scale?
Jim Yowell, Spring, Texas
The favorite candidate for dark matter, particles called WIMPs, would be matter unlike anything known on Earth. The WI in WIMP stands for “weakly interacting,” meaning there would be no normal interaction, or friction, with ordinary matter. — Tom Siegfried
“In the dark” (SN: 4/23/11, p. 24) describes the WIMP as possibly weighing “as much as 1,000 times the mass of the proton.” Is it possible that the dark matter particle is just the opposite: 1,000 times less massive than the proton, and that there are just lots more to account for the “missing” mass? Might not this explain why it hasn’t been detected?
Bill Robertson, Brookline, Mass.
There have been proposals that dark matter might consist of very light particles (one example is called the axion). Efforts to detect such light particles have also so far been unsuccessful. — Tom Siegfried
I found it interesting that “Out of the fabric” (SN: 4/23/11, p. 28) mentioned the idea of space becoming meaningless at very short (Planck length) distances, yet clung to the notion of time in such small places (“even if space is emergent, time may remain fundamental”). I have felt that both space and time are meaningless at such distances, and for the same reason: Measurements less than Planck length or Planck time — either one — are meaningless.
Before the Big Bang the universe existed entirely within that small space (or more accurately perhaps within a place with no dimensions, so the concept of size becomes meaningless). After the Bang both time and space sprang into existence at the moment matter was created, but the vestiges of the pre-Bang’s lack of space and time live on in the photon, the only entity today that could have existed pre-Bang. Time for a photon, traveling at the speed of light, does not pass in its frame of reference, and therefore time does not exist for that photon — just the same as before the Big Bang (hence, the photon’s infinite life). Time has not passed for photons traveling from the background radiation and arriving on Earth today, and from the photon’s perspective it is simultaneously everywhere (and nowhere) along the length of its travels since, for it, there is no time. For that photon, it is as if the Big Bang never occurred.
Time appears to be needed only because matter cannot travel at the speed of light, and time (and its corollary cause and effect) therefore emerges as a property of matter. Perhaps in the distant universe, when all matter decays to photons once again, there will be no concept of space and time once again. And perhaps that is all that is needed for another Big Bang: the absence of space and time. The article mentions the need to preserve time to explain cause and effect, but if the entire universe is pure energy — nothing but photons — then cause and effect become meaningless and time becomes equally meaningless.
Wally Magathan, Miami, Fla.
I love science. But tell me, what isn’t being said when in the April 23 issue you report the value of Hubble’s constant to be 70.4 (+1.3/–1.4), after having reported in a previous issue (SN: 4/9/11, p. 16) that this same constant has the value of 73.8 (with 3.3 percent uncertainty)?
T.B. Knost, Mills River, N.C.
These measurements aren’t as different as they appear. The uncertainty for the new Hubble constant measurement reported in the April 9 issue (plus or minus 3.3 percent) gives a range of 71.4 to 76.2, which overlaps with the previously established value of 69.0 to 71.7. This 69.0 to 71.7 range represents a consensus from measurements using many different methods. — Elizabeth Quill