1. ESo Santiago October 2003 Cosmology with Type Ia Supernovae

2. Supernovae are classified spectroscopically. If a SN has hydrogen emission lines in its spectrum, it is a Type II SN. Type II SN-e are single, massive stars (8 solar masses or more), which have evolved to become supergiant stars, then explode when their iron cores can no longer support the weight of the other layers. All the naturally occurring elements heavier than iron are produced by Type II SN-e.

3. Type Ia supernovae are considered to be explosions of C-O white dwarfs that reach the Chandrasekhar mass due to mass transfer from a nearby donor star. Their spectra are distinguished by the presence of Si absorption and a lack of hydrogen emission. The maximum brightness (and rate of decline) of the light curves is related to the amount of radioactive 56 Ni produced in the explosion. This decays into cobalt, then into iron. Models that work best to reproduce spectra and light curves involve a subsonic explosion (a deflagration) followed by a slightly off-center delayed detonation (Hoeflich et al. 2002).

4. SN 2004S at optical wavelengths. Krisciunas et al. (2006)

5. SN 2004S at near-IR wavelengths. Krisciunas et al. (2006)

6. Emissivity of a Co-Fe-Ni gas of a Type Ia SN as a function of ejecta temperature. Kasen (2006) 40 days after explosion...

7. Kasen (2006) has shown that the secondary hump in the near-IR light curves of Type Ia SN-e is caused by a redistribution of energy to longer wavelengths as the ejecta undergo a transition from doubly ionized iron-group species to singly ionized ones. There is even (marginal) evidence for a third maximum, corresponding to the transition from singly ionized to neutral species.

8. Late time behavior of SN 2004S.

9. Spectra of two “clones” at maximum light. No wonder their light curves are so similar!

10. Decline rate relations show shallower slope from shorter to longer wavelength. Krisciunas et al. (2003) Are SN Ia standard candles in the near-IR?

11. JHK templates for Type Ia supernovae at maximum light, constructed with stretch factors based on 1/(S B ) and 1/(S V ) Krisciunas et al. (2004b) RMS errors are 0.06 to 0.08 mag.

12. Hubble diagrams in the near-IR Except for SN 1991bg and SN 1999by, they are like beads on a string. Krisciunas et al. (2004c)

13. Except for the fastest decliners, Type Ia SN-e are standard candles in the near-IR.

14. Optical light is dimmed more than infrared light by interstellar dust.

15. V-band extinction A V = R V E(B  V), but not all interstellar dust behaves the same way. Cardelli, Clayton, and Mathis (1989)

16. Dust in the host of SN 2001el. A much more robust solution is obtained using optical and IR photometry. Krisciunas et al. (2006)

17. “If I had it to do over again, I would have become an astronomer, but only if R were constant.” -- Walter Baade (1953)

18. Riess et al. (1998) and Perlmutter et al. (1999) provided evidence from Type Ia supernovae that the expansion of the universe was accelerating. Science rated this the “discovery of the year”. What's a straightforward way to understand this finding?

19. If gravitational attraction of the all the galaxies is the only form of energy governing the expansion of the universe, then there is some “critical density” which will cause the expansion of the universe to slow down and eventually halt. Then the universe could recollapse. The critical density is given by  c = (3 H 0 2 / 8  G ) Astronomers refer to the ratio of the actual mean density of the universe to the critical density as follows:  matter =  c

21. Consider the “effective distance” (or proper motion distance, in Mpc): For an empty universe (  M = 0):

22. The observed flux (F) is related to the luminosity L, effective distance, and redshift as follows: The first factor of (1+z) arises because photons produced at frequency are observed at frequency /(1+z). We need a second factor of (1+z) because of time dilation of the arrival of the photons. We may thus define the “luminosity distance” to be:

23. The distance modulus is related to the luminosity distance by the standard equation... where the factor of a million is used because cosmic distance is commonly measured in Mpc, not pc. For the Einstein-de Sitter universe (  M = 1) d lum increases as z + ¼ z 2. For the empty universe, d lum increases as z + ½ z 2. For  M = 0,   --> 1, d lum increases as z + z 2. So the loci fan out in the Hubble diagram (plot of m  M vs. z).

24. Depending on  M and  , the loci are above or below the empty universe line in the Hubble diagram.

25. The high redshift supernova group at Lawrence Berkeley Lab was in the lead when it came to doing cosmology with high redshift supernovae. The next plot shows their data for 42 objects.

27. This is pretty convincing. Their data are consistently different than one would expect compared to the universe having a density equal to the critical density. However, half of their objects were observed in only photometric band, and thus had no information on dust extinction along the line of sight. Furthermore, 8 1/2 years later they still have not made the photometry of the individual objects available to the rest of the world to analyze independently. Did our group do any better?

28. Here are two ways to display the same set of data – either plot the data points as Y vs. X, or plot the differences of the data points and the model predictions.

29. Differential Hubble diagram (here using empty universe model as reference) makes it easier to see the trends in the actual data.

30. On the basis of what we knew about the structure of the universe, astronomers working on distant supernovae expected that the data points would fall along the line called “open”. That is because they believed that the average mass density of the universe was about 30 percent of the critical value to cause the universe's expansion to halt. However, when they plotted the actual data points, the supernovae were “too faint” by about 0.25 magnitudes. Was it due to calibration errors? Are supernovae at high redshift the same as nearby supernovae? Have we properly corrected for dust in the host galaxies of the distant supernovae?

31. The Essence Project ● 6 year project using CTIO 4-m + mosaic camera ● Oct-Dec, 10 half-nights per lunation (every other night) ● Spectra of candidates using Keck, VLT, Gemini, Magellan ● Goal is 200 Type Ia supernovae, redshift 0.2 to 0.8 ● Attempting to determine the w = p/(  c 2 ) to +/- 10% During the first five years of the project we have found a total of 171 Type Ia supernovae. For radiation w =+1/3. For cosmological constant w =  1.

33. Our supernovae have look-back times of 2 to 7 billion years.

34. Krisciunas et al. (2005) 9 ESSENCE SN-e discovered in 2003 and observed with HST. Type Ib/c? Redshift unknown Why do so many of these have such faint hosts? Redshifts from 0.53 to 0.79

35. Krisciunas et al. (2005) Squares = ground- based data (larger error bars) Triangles and dots = HST data

36. Riess et al. (2004) If we put the data into redshift bins, this is what the differential Hubble diagram looks like, including objects beyond z = 1 discovered with HST. If we look far enough back in time, we can detect Type Ia SN-e that exploded before the era of cosmic acceleration. But...

37. Krisciunas et al. (2005) A differential Hubble diagram plotting all the individual points is not quite as convincing.

38. A combination of data indicates that the mass density of all matter amounts to 30 percent of the critical density, and that the cosmological constant makes the geometry of the universe flat.

39. The future: Rest frame optical AND infrared light curves, which involves ground-based IR obss with the world's largest telescopes, or space-based obss with HST, Spitzer, and future satellites. Redshift 0.43 SN observed in J-band with Magellan 6.5-m in December of 2004.

40. Was also observed with Spitzer Space Telescope. Redshift 0.458 SN discovered in Dec. of 2005.

41. Conclusions At near-IR wavelengths Type Ia SN-e are standard candles at maximum light (except for the fastest decliners). This makes them excellent tools for extragalactic distances, and IR obss lead to values for host galaxy extinction whose systematic errors are of order the photometric errors. In 6 observing seasons we should find roughly 200 Type Ia SN-e with redshifts between 0.2 to 0.8. This should allow a determination of the equation of state parameter to +/- 10 percent. The present data (CMB, large scale structure, SN-e) are consistent with flat geometry and dark energy equivalent to Einstein's cosmological constant.