Distances to supernovae are measured using Hubble’s law (red shift/magnitude relation).

1. Observations of distant supernovae and fluctuations in the cosmic microwave background indicate that the expansion of the universe is accelerating. Astronomers theorize that this acceleration is caused by “Dark Energy”.

2. Distances to supernovae are measured using Hubble’s law (red shift/magnitude relation). • Distant supernovae are less luminous than expected. • Expansion of the universe is accelerating. • One explanation: dark energy drives the acceleration.

3. What is dark energy? • We don’t exactly know … but we can infer its properties from observations: • it is evenly distributed through space • it makes up about 70% of the universe Matter we can observe directly Non-luminous matter in galactic halos

4. Dark energy should not be confused with dark matter. Dark matter is non-luminous material detected through its gravitational effect on galaxies and galactic clusters.

5. The solid blue line shows a theoretical prediction for a universe with 70% dark energy. Most supernovae have brightness and red shift values that lie close to this line. Supernova brightness Red shift

6. More evidence for dark energy comes from high precision maps of the cosmic microwave background (CMB) radiation measured by the WMAP satellite.

7. WMAP data was used to create a statistical model of the “lumpiness” of the cosmic microwave background. • Theory predicts how the statistics depend on the presence of dark energy. • Results agree with dark energy hypothesis and supernova measurements.

8. This graph shows a spectrum of CMB temperature variations with direction (anisotropies) as measured by WMAP. The vertical axis is related to the CMB temperature fluctuation. The horizontal axis is the angular scale on which the fluctuation occurs.

9. Various measurements are combined to “zero in” on the answer: Plot the range of dark energy density (ΩΛ) versus matter density (ΩM, dark+visible) predicted by the measurement. The sum of these adds up to about 1.02, the flat universe density measured by WMAP, so the answer lies along the dashed line and its intersection with Supernova (SN) data. Best fit: 73% dark energy, 27% dark + ordinary matter

10. However, other recent findings cast doubt on dark energy interpretations. First, XMM-Newton X-ray telescope studies of galaxy clusters show inconsistencies with WMAP data. The X-ray Multiple-Mirror (XMM) Telescope

11. The XMM-Newton • x-ray telescope has observed distant galaxy clusters to emit more x-rays than expected. • Results are inconsistent with WMAP analysis, implying more matter and less dark energy. XMM-Newton X-ray telescope image

12. Second, reanalysis of WMAP data to include the Sunyaev-Zel’dovich Effect exposes another inconsistency. Observation of galaxy clusters applying the Sunyaev-Zel'dovich Effect

13. The Sunyaev-Zel’dovich Effect: • Microwaves scatter off electrons in hot gas clouds and gain energy. • Depending on how these gas clouds are distributed in space, they can distort characteristics of the CMB. • Ongoing research may determine if this effect significantly impacts dark energy measurements.

14. Exciting young areas of science are often controversial. Stay tuned as additional data, improved analysis, and new theoretical frameworks help astronomers resolve dark energy questions in the future.