Goal: To understand voids Objectives: 1)To examine the Size and distribution of voids 2)To understand the Properties of voids 3)To learn about the Formation and evolution of voids

What is a void? A) A region with no gas B) A region with no dark matter C) A region with no galaxies D) None of the above

Size and distribution of voids Voids are regions of space with less galaxies. Voids cover over half of the volume of our universe!

Quick philosophy Everything is connected in someway – even virtually nothing. As we will find, we can still find out a lot about our universe by looking at areas where there is very little. The moral of the story is to never (in either life or Astronomy) take anything forgranted.

Nearest voids

Universe And with time the voids will grow! 1) Clusters will shrink 2) Universe will expand

Properties of voids Voids – much like space – is not 100% empty. There are still galaxies, but there are only 20% of them (1/5 th normal). Voids are about million light years across. The galaxies seem like normal galaxies. They have normal amounts of metals. They do tend to have a stronger old population but have normal amounts of metals. The only real difference is the distribution of galaxy types.

Void galaxies Void galaxies are not as you expect though. They seem to be brighter than average. There seems to be a larger star formation rate than average (which makes them bluer). The only non surprise is a lack of spiral galaxies (10X lower gas makes it hard to have a spiral galaxy). Spirals tend to reside on the edges of galaxy clusters or in smaller galaxy groups by themselves. Basically spirals live in the suburbs not the rural areas.

Problems with observations However, these studies only include void galaxies (compare that to the estimated trillions of galaxies in the universe). Could we only be seeing the special galaxies that have had something strange happen to them (such as a merger)?

More problems Surveys of galaxies (voids and non void) are limited by some brightness. There are usually a lot of galaxies dimmer than can be observed in any region of space any distance from us. So, we don’t get a complete map. This affects where we observe the voids to start and stop. We can’t measure distances directly. We have to use redshifts. However, galaxies on the edges of voids tend to move AWAY from the voids. This tends to change our view of where these galaxies are and what the shape of the void is (elongates them). They are also so big that the Cosmological constant (the change in the rate that the universe expands) can also change our viewed shape of the voids in the same fashion. Finally, the surveys only go so far. So we can only observe the properties of the most nearby voids.

Unresolved questions Where is the gas coming from for the star formations? We expect the gas to leave the void and go to the clusters (and voids end up with only 10% of the normal gas amounts). Could the clusters be pumping gas back into the void? Is there more dark matter than expected in the void?

A NEW issue! A group at the University of Minnesota discovered a void a BILLION light years across! This is something we just cannot explain currently as we expect them to be million light years in size.

So, why bother studying voids? What can we learn from voids, why are they important? The size and distribution of the voids eliminates any possibility that the dark matter in our universe is warm or hot (it would spread out too much). Therefore the dark matter is cold. The sizes and distributions of the voids also show how much mass our universe contains.

Also learn Since the voids form as a result of the distributions of dark matter in the formation of the universe therefore the sizes and distributions tell you something about the distribution of dark matter when the universe formed! Also, since the curvature of the universe (aka the cosmological constant) changes the shapes of voids as we see them, we can gain some understanding of what the curvature of the universe is (and with less bias than other methods).

And… Some final things we gain are: Constraints on galaxy clustering (more clustering means larger voids, less means smaller). Sheds some insight into galaxy evolution. It is amazing how much we can learn from looking at places with so little.

Conclusion Studying voids teaches us a lot about the universe and the evolution of galaxies. Voids are not 100% empty. Galaxies, while scarce are teaming with new stars – which is a great mystery. The bottom line though is that most of our universe (volume wise) is made of voids, so they have to be important.