In macroscopic organisms, aging is often obvious; in single-celled organisms, where there is the greatest potential to identify the molecular mechanisms involved, identifying and quantifying aging is harder. The primary results in this area have come from organisms that share the traits of a visibly asymmetric division and an identifiable juvenile phase. As reproductive aging must require a differential distribution of aged and young components between parent and offspring, it has been postulated that organisms without these traits do not age, thus exhibiting functional immortality. Through automated time-lapse microscopy, we followed repeated cycles of reproduction by individual cells of the model organism Escherichia coli, which reproduces without a juvenile phase and with an apparently symmetric division. We show that the cell that inherits the old pole exhibits a diminished growth rate, decreased offspring production, and an increased incidence of death. We conclude that the two supposedly identical cells produced during cell division are functionally asymmetric; the old pole cell should be considered an aging parent repeatedly producing rejuvenated offspring. These results suggest that no life strategy is immune to the effects of aging, and therefore immortality may be either too costly or mechanistically impossible in natural organisms.

Aging, defined as a decrease in reproduction rate with age, is a fundamental characteristic of all living organisms down to bacteria. Yet we know little about the causal molecular mechanisms of aging within the in vivo context of a wild-type organism. One of the prominent markers of aging is protein aggregation, associated with cellular degeneracy in many age-related diseases, although its in vivo dynamics and effect are poorly understood. We followed the appearance and inheritance of spontaneous protein aggregation within lineages of Escherichia coli grown under nonstressed conditions using time-lapse microscopy and a fluorescently tagged chaperone (IbpA) involved in aggregate processing. The fluorescent marker is shown to faithfully identify in vivo the localization of aggregated proteins, revealing their accumulation upon cell division in cells with older poles. This accretion is associated with >30% of the loss of reproductive ability (aging) in these cells relative to the new-pole progeny, devoid of parental inclusion bodies, that exhibit rejuvenation. This suggests an asymmetric strategy whereby dividing cells segregate damage at the expense of aging individuals, resulting in the perpetuation of the population.ibpA ͉ inclusion bodies ͉ protein aggregation ͉ small heat-shock protein

We construct, for the first time, a model of graceful exit transition from a dilaton-driven inflationary phase to a decelerated Friedman−Robertson−Walker era. Exploiting a demonstration that classical corrections can stabilize a high curvature string phase while the evolution is still in the weakly coupled regime, we show that if additional terms of the type that may result from quantum corrections to the string effective action exist, and induce violation of the null energy condition, then evolution towards a decelerated Friedman−Robertson−Walker phase is possible. We also observe that stabilizing the dilaton at a fixed value, either by capture in a potential minimum or by radiation production, may require that these quantum corrections are turned off, perhaps by non-perturbative effects or higher order contributions which overturn the null energy condition violation.

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We consider the problem of constructing a non-singular inflationary universe in stringy gravity via branch changing, from a previously superexponentially expanding phase to an FRWlike phase. Our approach is based on the phase space analysis of the dynamics, and we obtain a no-go theorem which rules out the efficient scenario of branch changing catalyzed by dilaton potential and stringy fluid sources. We furthermore consider the effects of stringloop corrections to the gravitational action in the form recently suggested by Damour and Polyakov. These corrections also fail to produce the desired branch change. However, focusing on the possibility that these corrections may decouple the dilaton, we deduce that they may lead to an inflationary expansion in the presence of a cosmological constant, which asymptotically approaches Einstein-deSitter solution.Submitted to Nuclear Physics B theory are imminent. In most cases, the route taken was minimalistic in philosophy, consisting of inventing and incorporating changes to account for some of the phenomenologically dictated mechanisms. This approach has been only partially satisfactory, failing to produce a model capable of dealing comprehensively with the shortcomings of Einstein's theory, which proved deserving of respect even in its demise.On the other hand, the ongoing quest for the unified theory of interactions in Nature has finally produced a promising contender, which has withstood theoretical scrutiny up to date. The advent of string theory [3], based on a fundamentally different assumption about the nature of matter, has been supported mainly by the description of gravity in a manner equivalent to other forces, and perhaps even more importantly, by the absence of some of the 1 obstacles encountered in the failed attempts to quantize General Relativity. Thrusting from the initial success, many studies have sprung up investigating gravitational aspects of string theory, and in particular the early universe cosmology [4]- [20]. The justification for this interest can be naturally found in the fact that string theory, while claiming to unify gravity with the other forces of nature, must give us the means to investigate the regions where the standard model has failed to give satisfactory answers. The aforementioned cosmological problems, especially the problem of the initial singularity, fall precisely in this category. In addition, results of these investigations should provide us with ways to test the compatibility of string theory with the Nature.While the early investigations of string cosmology have indicated the presence of some of the coveted mechanisms to tackle the encountered problems, they have also burdened us with a host of new difficulties. In a typical cosmological setting, most of the advantages and the difficulties can be attributed to the presence of a new field, the scalar dilaton, which comes in response to the requirement of conformal invariance of string world-sheet theories. The dilaton has been recognized as a natural candidate for the inflato...

We re-examine the graceful exit problem in the pre-Big Bang scenario of string cosmology, by considering the most general time-dependent classical correction to the Lagrangian with up to four derivatives. By including possible forms for quantum loop corrections we examine the allowed region of parameter space for the coupling constants which enable our solutions to link smoothly the two asymptotic low-energy branches of the pre-Big Bang scenario, and observe that these solutions can satisfy recently proposed entropic bounds on viable singularity free cosmologies.

We determine the top quark mass m t using t t pairs produced in the DO " detector by ͱsϭ1.8 TeV pp collisions in a 125 pb Ϫ1 exposure at the Fermilab Tevatron. We make a two constraint fit to m t in t t→bW ϩ b W Ϫ final states with one W boson decaying to qq and the other to e or . Likelihood fits to the data yield m t (lϩjets)ϭ173.3Ϯ5.6 (stat) Ϯ 5.5 (syst) GeV/c 2 . When this result is combined with an analysis of events in which both W bosons decay into leptons, we obtain m t ϭ172.1Ϯ5.2 (stat) Ϯ 4.9 (syst) GeV/c 2 . An alternate analysis, using three constraint fits to fixed top quark masses, gives m t (lϩjets)ϭ176.0 Ϯ7.9 (stat)Ϯ 4.8 (syst) GeV/c 2 , consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented. ͓S0556-2821͑98͒06815-5͔

String cosmology solutions are examined in a generalized phase-space including sources representing arbitrary corrections to lowest order string-dilatongravity effective action. We find a set of necessary conditions for a graceful exit transition from a dilaton-driven inflationary phase to a radiation dominated era. We show that sources allowing such a transition have to violate energy conditions similar to those appearing in singularity theorems of general relativity. Since familiar classical sources, excepting spatial curvature, obey these energy conditions we conclude that a generic graceful exit in string cosmology requires a new effective phase of matter. Our results clarify and generalize previous analyses and enable us to critically reexamine proposed non-singular cosmologies.