A test of the nature of cosmic acceleration using galaxy redshift distortions.

Observations of distant supernovae indicate that the Universe is now in a phase of accelerated expansion the physical cause of which is a mystery. Formally, this requires the inclusion of a term acting as a negative pressure in the equations of cosmic expansion, accounting for about 75 per cent of the total energy density in the Universe. The simplest option for this 'dark energy' corresponds to a 'cosmological constant', perhaps related to the quantum vacuum energy. Physically viable alternatives invoke either the presence of a scalar field with an evolving equation of state, or extensions of general relativity involving higher-order curvature terms or extra dimensions. Although they produce similar expansion rates, different models predict measurable differences in the growth rate of large-scale structure with cosmic time. A fingerprint of this growth is provided by coherent galaxy motions, which introduce a radial anisotropy in the clustering pattern reconstructed by galaxy redshift surveys. Here we report a measurement of this effect at a redshift of 0.8. Using a new survey of more than 10,000 faint galaxies, we measure the anisotropy parameter beta = 0.70 +/- 0.26, which corresponds to a growth rate of structure at that time of f = 0.91 +/- 0.36. This is consistent with the standard cosmological-constant model with low matter density and flat geometry, although the error bars are still too large to distinguish among alternative origins for the accelerated expansion. The correct origin could be determined with a further factor-of-ten increase in the sampled volume at similar redshift.

A large population of galaxies 9 to 12 billion years back in the history of the Universe.

To understand the evolution of galaxies, we need to know as accurately as possible how many galaxies were present in the Universe at different epochs. Galaxies in the young Universe have hitherto mainly been identified using their expected optical colours, but this leaves open the possibility that a significant population remains undetected because their colours are the result of a complex mix of stars, gas, dust or active galactic nuclei. Here we report the results of a flux-limited I-band survey of galaxies at look-back times of 9 to 12 billion years. We find 970 galaxies with spectroscopic redshifts between 1.4 and 5. This population is 1.6 to 6.2 times larger than previous estimates, with the difference increasing towards brighter magnitudes. Strong ultraviolet continua (in the rest frame of the galaxies) indicate vigorous star formation rates of more than 10-100 solar masses per year. As a consequence, the cosmic star formation rate representing the volume-averaged production of stars is higher than previously measured at redshifts of 3 to 4.

Helicobacter pylori prevalence in acquired immunodeficiency syndrome.

Helicobacter pylori is consistently reported with high prevalence in HIV-negative patients with chronic gastritis and active ulcer disease. This study is an evaluation of the prevalence of H. pylori in AIDS patients, and the association with chronic gastritis, erosions, and ulcer disease. Seventy-three AIDS patients referred for the evaluation of gastrointestinal symptoms underwent upper endoscopy and antral gastric biopsy. Histologic gastritis was diagnosed and degree of activity graded on hematoxylin-eosin stain. H. pylori organisms were identified by acridine orange stain. A single pathologist evaluated the biopsy specimens. H. pylori was found in 15% (11 of 73) of AIDS patients. Histologic chronic active gastritis was evident in 94.5% (69 of 73) of the study group. H. pylori was identified in 15.9% (11 of 69) of biopsy specimens with histologic chronic active gastritis. The organism was more common in biopsy specimens with a higher grade of activity in the chronic gastritis. Endoscopic erosions or ulcers were noted in 11 patients (seven gastric, four duodenal). H. pylori was present in 18% (2 of 11) of AIDS patients with erosions or ulcers. The prevalence of H. pylori in AIDS patients with histologic chronic active gastritis is much lower than the prevalence previously reported for HIV-negative patients with similar pathology. The low prevalence observed does not implicate H. pylori as the causal agent in most chronic active gastritis in the AIDS population. Impaired acid secretion may reduce colonization of gastric mucosa and explain the low rate of H. pylori observed.

Lower Helicobacter pylori infection and peptic ulcer disease prevalence in patients with AIDS and suppressed CD4 counts.

OBJECTIVE: The prevalence of Helicobacter pylori (HP) has previously been reported to be lower in AIDS patients. This study evaluated the prevalence of HP and peptic ulcer disease in relation to absolute CD4 counts in HIV-seropositive patients with GI symptoms. DESIGN: Seventy-two patients (48 HIV-positive and 24 HIV-negative) with GI symptoms were evaluated with upper endoscopy and antral gastric biopsy. Samples were prepared with Giemsa stain and reviewed by a single pathologist to determine status of HP infection. The patients were stratified on the basis of HIV status and CD4 count: group A, HIV-positive patients with a CD4 count greater than 200, group B, HIV-positive patients with CD4 counts less than 200, and group C, an HIV-negative control group. RESULTS: The prevalence of HP infection in the three groups was as follows: group A 69% (11/16), group B 13% (4/32), and group C 63% (15/24). Peptic ulcer prevalence in group A was 19% (3/16), group B 3% (1/32), and group C 25% (6/24). CONCLUSIONS: The prevalence of HP in HIV-positive patients with a CD4 count less than 200 is significantly lower (p < 0.001) than that found in HIV-negative patients. The number of peptic ulcers in the HIV-positive group with CD4 < 200 was significantly less (p = 0.035) than that of the HIV-negative patients. These results suggest a role of CD4 cell and immune function in sustaining HP infection and HP-related peptic ulcer disease.