The light and the dark

In, say 1975, there was only one kind of 'stuff' in the Universe, matter made of protons and neutrons. By 1985 there was a consensus that this was not enough, the gravity of the stars in galaxies was not sufficient to hold them together. Something else was there, vast, mysterious and imperceptible, only observable by its effects on its surroundings. Imagine HG Wells 'Invisible Man' moving unseen through a room until he knocks over a vase. There was much more of this dark matter out there than stuff we could experience directly.

By 2005 it was clear that there was more going on out there. Another unseen and unknown something was pushing the galaxies apart. This is dark energy, a force which permeates all space. Astonishingly weak, dark energy can still move galaxies given aeons of time.

No one knows what dark energy and dark matter are; researchers study the light of far off galaxies to see how these elusive quantities effect them. The image shows the galaxy cluster Abell 1689, light from distant galaxies is bent by the gravity from galaxies closer to us but on the same line of sight creating a text-book example of a gravitational lens. This gravity is mainly, we now know, exerted by the dark matter in the galaxies (the blue fuzz in the image is not real, it is a superimposed map of matter distribution). At the same time, however, dark energy is pushing the galaxies apart adding a subtle additional distortion to this ancient light.

Dark matter and dark energy are utter mysteries today but so were electricity, magnetism and radioactive decay once. In some future time we will understand dark energy and matter just as well. Dare I speculate that one day we shall apply them? (I'm holding out for spindizzies myself.)

Image credit: NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM).

Cosmic Coelacanths

We all love bright and showy spiral galaxies! So much so, that we tend to overlook the elliptical galaxies which make up about 30% of the galaxies out there.

Smaller than spiral galaxies, elliptical galaxies may be full of stellar living fossils, surviving virtually unchanged from an older cosmic era. Their stars are mainly old reddish stars, rather than the young blue-white star which blaze in the arms of spiral galaxies. This aging population is because there an initial frenzy of star formation in the galaxies' early days this soon fizzled out. The interstellar matter which provided the raw material for new generations of stars in galaxies like our own is scarce in elliptical galaxies. This absence of gas and dust explains an elliptical galaxy's spheroidal shape; there is nothing to flatten the orbits of the stars into a single plane of rotation.

Elliptical galaxies are smaller than spiral galaxies, roughly spherical and full of old stars. You may be thinking that this sounds familiar and you would be correct. Elliptical galaxies are very similar to the cores of spiral galaxies. This may be exactly what they are. We know that galaxies interact and even collide. Theories suggest that this processed can rip the arms clean off spiral galaxies, leaving behind a disrupted core which we see as an elliptical galaxy. What happens next? Perhaps the galaxy will accumulate gas and dust from the intergalactic medium as it wanders through the void, building new spiral arms in the process. Alternatively it will simply drift on forever as a cosmic coelacanth, reminding observers of the Universe's earlier days.

The image (click on it to enlarge it) shows elliptical galaxy NGC 4696 in the Centaurus Galaxy Cluster, notable for the 30 000 light year long streak of dust running across it. NGC 4696 is some 155 million light years from our own galaxy, so we are seeing it with light which left the galaxy in Earth's early Cretaceous Period, about the time the first mammals were scurying between dinosaurs' feet.

Image Credit: ESA/Hubble and NASA