Re: How do we know that objects seen in Hubble Deep Field are all the same age?

5/23/00

Dear Cynthia,

The complex interdependency between fact and theory in the progress of science is often mis-understood. The two words seem simple: theories are ideas that can be wrong, and facts cannot--right? In reality, scientific theories are not just single ideas that are either assumed or not assumed. Scientific theories, like the (standard model of particle physics, general relativity, evolution by natural selection, or the big bang theory) are precise sets of ideas which, when logically followed, make many detailed predictions about what we should see in nature (the facts). You're right that if experiments show those predictions to be false above and beyond the uncertainties and errors in experimental methods, the theory loses its credibility and goes off into history like the famous "Ether" of 19th century physics.The truth is that the theories I listed above have accurately predicted the results of so many thousands of carefully peer-reviewed published experiments, have made so many disparate results come together into frameworks that makes sense, that they've downright earned the credibility you give to an old friend who has stood by you through thick and thin and whose advise has never been wrong.

The Big Bang is a theory because it's a precise set of ideas which makes predictions that are explorable through experiment. That doesn't mean that it hasn't already been confirmed by many different experiments over and over and over again, throughout decades of testing. It's a theory, and it's true, because nature has done nothing but back it up. If we were to suddenly, today, decide to throw out the Big Bang theory, It would be impossible to go back through all of the experiments and find some other self-consistent logical way to make them all make sense together.

The Hubble Deep Field question is somewhat separate. If I understand what you're saying correctly, one cannot just assume that two objects next to each other in any picture of the sky are the same age. I grant you that that is a pretty difficult hypothesis to prove. What the study you saw might be saying (I'm actually not sure what study you're discussing, so I'm not terribly sure I can do justice to the authors here) is that a group of galaxies which right next to each other in the Hubble Deep Field and having similar brightness are likely to be gravitationally bound to one another and thus to have formed at about the same epoch.

The brightness issue is important. The reason the Hubble Deep Field is "deep" is because they collected light from the same little patch of sky for so long that even the very dimmest objects showed up. There are two ways to be dim in a picture: you can be incredibly far away and small, or you can be dim, big, and close by. Distance could be demonstrated to those galaxies with redshifts, but since spectra require a certain brightness of the object, I'm not sure if the authors have been able to do that. Proving that they're at about the same distance would help to show that they're close, and so are interacting gravitationally. If the cluster is sufficiently far away, we're seeing it as it was not so long after matter first began to clump together.

Using redshifts to get the distances to the galaxies in the Hubble Deep Field is about the only way the Big Bang could get involved in this problem. Line-of-sight velocity of a galaxy is related to its distance from us by Hubble's Law, v=H*d, where H=70+/-10km/s/Mpc. Actually, it was the empirical discovery of Hubble's Law that first made clear that the universe is expanding. Hubble simply made a plot of the distances to galaxies (found by independent methods) vs. velocities along their line-of-sight. Low and behold, it was a straight line! That is exactly how things behave which have been blown up in an explosion.

I hope this answers at least parts of your questions. For the record, I'm really glad you asked about the nature scientific theory and fact. It's challenging to think about how ideas come to be trusted under a methodology which, by definition, never unconditionally trusts anything. In my limited experience so far, it's like a great marriage or friendship. If it survives decades of challenges and tests and still provides meaning, then even though it doesn't stop being tested you feel increasingly safe building upon it and trusting it.

Thanks,

Kristin Nelson-Patel :)