Expert: Courtney Seligman - 1/6/2010

Question
Hello,

I am a biology major but I've always been fascinated with astronomy, too.  I've been following the Kepler mission very closely and I've read the news about the new Hubble images that have peered back to 13.2 billion years ago.

I understand the concept of a light year and how looking over a distance across space is actually looking back in time and I know light travels at around 300,000 km/s.  Still, even if we know how fast light travels, how do scientists determine whether an object we're seeing is ten light years away or ten billion light years away?  Is there some kind of reference point we use?

Tom  

Answer
For distances such as 13.2 billion light years, we use the assumption that the rate of expansion of the space between us and the distant object is some constant rate (which has to be determined in some other way). If that is correct, then the faster the recessional velocity of the object (determined by the redshift of its spectral lines), the further away it is assumed to be (or more accurately, the longer the light that we now see took to make its way through the expanding space between it and us). (You might find it useful to refer to Light Travel Times and Cosmic Distances, at http://cseligman.com/text/galaxies/lighttravel.htm for a simplified discussion of how this light-travel time compares to the actual distance of the object.)

Whether the statement above is correct has to be determined by comparing the recessional velocity of bright distant objects with their actual distance, determined in some other way. The best way to do this at the moment is to study the brightnesses of type Ia supernovae. Such supernovae occur when a white dwarf has mass dumped onto it by a dying companion, and as a result, blows itself to smithereens (see Mass Transfer In Binary Star Systems, at http://cseligman.com/text/stars/binary.htm). About a decade or so ago, it was discovered that the "light curve" of the resulting supernova has a fairly uniform nature, and that correcting for differences in the light curves of such supernovae yields a surprisingly accurate measure of their true brightness. Comparing that to their apparent brightness allows us to calculate their distance, and determine the expansion rate of space at various times in the past. That turns out NOT to be a constant -- it was faster very early on, slower later on, then faster in more recent eons -- but given that knowledge of the changing rate of spatial expansion at various times, we can calculate how long it took light to reach us, for any observed recessional velocity.

Prior to the discovery of this property of type Ia supernovae, estimates of the expansion rate of the Universe at various times differed by as much as a factor of two, so different studies gave very different estimates of the distance of distant objects. Now, we are more certain of how long it took the light from those objects to reach us, but both in the past and the present, the recessional velocity is/was what is/was measured, and the corresponding light travel time is/was calculated from that, rather than being directly measured.

As a result, if you read that one object is 13 billion ly away, and another 13.2 billion ly away, the numbers might be (hopefully only slightly) "off", but the more distant object is almost certainly more distant, regardless of what the actual distances are. (As an example, in a very distant cluster of galaxies, some galaxies would be coming toward us, and others going away from us -- in addition to their overall motion away from us -- due to their motions relative to each other. The galaxies moving toward us, relative to the rest of the cluster, would be calculated as being closer; while the ones moving away from us, relative to the rest of the cluster, would be calculated as being more distant. Unless this is corrected for, the cluster would then appear to be a "line" of galxies, pointing toward us, for the galaxies moving in our direction relative to their companions, and aaway from us, for the galaxies moving away from us relative to their companions.)

I have to leave for a doctor's appointment in less than five minutes, and don't have time to carefully check this for typographical or grammatical errors; so please forgive any minor mistakes; and if you need a more careful explanation of any part of this discussion, please feel free to contact me again, after I return.)