The cover story of this month’s Discover magazine (not yet available online) brought to mind a question that has been bugging me for years. I’ve searched extensively, but been unable to find an answer. I’m hoping folks here (especially our physics types) can explain.
In a nutshell, the problem is this. We have two data sets. One is a set relating to the Hubble constant, the other relates to Type 1a supernova. The former shows a constant rate of expansion going back about 1.3 billion years (the limit of the data), presently estimated at something like 70 km/s/Mpc. The latter shows that the rate of expansion in the distant past, e.g., 5 billion years ago, was significantly less than we observe today. For reasons I’ve never understood, the latter is widely described as demonstrating that the universe’s current rate of acceleration is increasing (emphasis on present tense), as opposed to demonstrating only that it changed in the past (by definition, an acceleration, but not necessarily one that’s still occurring).
The difference is enormous. If the rate of expansion is increasing in the present tense, it should show up in the Hubble data. Conversely, the fact that it doesn’t show up there is strong evidence the rate isn’t currently increasing. Moreover, it isn’t hard to reconcile the two data sets. Suppose the Hubble expansion is basic and universal, but subject to gravity. This is scarcely surprising. It has always been recognized that Hubble flow only dominates on very large scales. Gravitationally bound systems, e.g., our solar system and the Milky Way, show no Hubble effects. Move to a middle ground, where galaxies aren’t gravitationally bound but not yet completely independent and one would expect just the result the Type 1a supernova data suggest.
To see this, consider a thought experiment. Imagine a long conveyer belt, like the people movers they use at airports. At one end of the belt, place a small robotic car driven by a photo-voltaic cell. Just off the belt, place a focused spotlight. The spotlight will power the cell, and hence the robotic car. Switch on both the belt and the spotlight. The belt takes the car away from the light; the light powers the car towards it. If the two actions are equal, the car stays in place. If the light is stronger, the car advances; if the belt is stronger, the car recedes. Our universe, based on the data, corresponds to the third case. That is, Hubble flow takes the car one way and gravity the other, with the former being stronger. Importantly, notice two things. First, the further the robotic car gets from the spotlight, the less power it draws and hence the less it is able to resist the movement of the belt. Second, as the power from the spotlight fades in significance, the car accelerates, i.e. goes faster, but the speed of the belt is a limit. Take the spotlight out of consideration entirely and the fastest the car will recede is the belt speed. Stated a little differently, observing a historical change in speed doesn’t necessarily imply acceleration in the present or future tense.
Now, I readily admit I’m not a physicist. Moreover, it won’t surprise me if there’s some simple answer to the question that I’ve overlooked. So, please, someone, explain it to me.
Notes:
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I give Wiki links above for the benefit of folks who might want to brush up quickly on the background of the issue. Naturally, these articles cover only the basics, but to cite all the sites I’ve read, e.g., here and here, would clutter up the thread to no real advantage. In fact, most of my understanding is based on an introductory college level astronomy text, The Universe by Freedman & Kaufmann, which I picked up used several years ago. It’s a bit dated - I have the 2005 edition - but have seen nothing in my other reading which suggests it has been rendered obsolete on the question I’m raising.
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One of the curious things about this topic is that almost everyone seems to regard the Type 1a supernova data as raising the question of how the universe is expanding. Except, well, no. That problem has been around since Hubble. Call it the cosmological constant. Call it dark energy. Call is quintessence. Call it the vacuum energy of space. Whatever. Why the universe expands is a question that has been with us for almost a hundred years.
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It’s easiest to understand the thought experiment given above if we assume the conveyer belt is moving at a constant speed. This isn’t quite analogous to Hubble flow, as that’s an acceleration, i.e., the rate of expansion increases with distance. This isn’t important for my purposes, as all I’m trying to demonstrate is how two forces operating in opposite directions can produce a changing acceleration yet terminate at a constant. If you prefer, assume the belt accelerates over time (or whatever you think best corresponds to Hubble flow). The effect of the reverse force falling off over time and/or distance is the same. In any event, let’s not get sidetracked on the conveyer-belt-keeping-a-plane-from-taking-off paradox. Yeah, it’s an obvious joke, but not relevant here.